It's a common topic since the Domizilla came out: which to get if you could only get one? It's also a very valid question since they aren't inexpensive.
With the recent offerings from Seneca Woodworking along with their new prototypes I reviewed recently, the choice becomes more muddied. So let's go over the differences, what they mean in real use, and comment on how each machine stacks up to different common projects so you can better identify with a certain machine's capabilities.
The first thing to point out is that nothing suggests the 700 is a replacement for the 500; they are distinctly different machines; both will be around a long time.
Here are the specifications we'll look at:
|Cutter Diameters||4, 5, 6, 8, 10mm||8, 10, 12, 14mm||4, 5, 6mm|
|Mortise Widths||exact, +6mm, +10mm||exact, +3mm|
|Fence Height||5-30mm||10-52mm||down to 1mm|
Let's look at the specifications individually...
For some, the weight of the machine can be a big deal. The 700 is 62% heavier than the 500. Generally, I care less about the weight than other specs on a tool, but I could see a reluctance to want to lug a tool 62% heavier than another that can do the same job. This is especially the case with a Domino where you are mortising many holes on different project pieces. Contrast this to a heavy router where you put it down and do a lot of work without lifting it again.
This is a big difference between the machines that often goes overlooked. Well, partially overlooked...
The 500 can plunge to 12, 15, 20, 25, and 28mm depths.
The 700 can plunge from 15 to 70mm in 5mm increments. While this can seem the same, remember that you don't have a 12mm equivalent. Sometimes you need a shallow plunge in one piece and deeper plunge in the mate to keep from piercing a side. Not a big deal, but something to consider.
Now the 700 ships with a plunge lock; it is a piece of plastic pushed over a plunge rod for shipping. Keep it; slice off a 2mm wide piece and keep it as a stop as explained in the 700's supplemental manual. With it, you can set your plunge to, say, 30mm and slip on the 2mm spacer to get an effective plunge of 28mm. If you created a 3mm wide spacer, you could set the plunge depth to 15mm with the spacer for an effective depth of 12mm. You've now used these jigs to duplicate the two mortise depths the 500 has that the 700 doesn't directly have.
With the depths provided with the stock 700, you could use any of the 500's tenon lengths since they are all multiples of 5mm in length (30mm, 40mm and 50mm).
Prior to the Seneca accessories, this specification was usually the deciding factor for people to choose one over the other. Now it is fuzzy. Gee, thanks, guys...
Prior to Seneca's bit adapter, the minimum 8mm diameter bit could be too large for some projects. Certainly for 1/2" (12mm) ply, that's gluing the tenon to basically the outside skins. Even for 3/4" (18mm) ply, 6mm is a better size though 8mm will work. Length-wise, you'd have a minimum length of 40mm for the pre-cut Dominos. Naturally cut-to-length stock (or home-made) could be made to match the 30mm minimum tenon length (using the 15mm minimum depth on both sides without any spacer).
With Seneca's bit adapter, all the 500's bits are available for the 700. Excellent news for dealing with thinner stock or when you just want a smaller tenon like the 5mm tenons for alignment of panel parts. Granted, the adapter does not extend the plunge depth of those bits: they are still limited to 28mm plunges, which with the 700's "multiples of 5mm" plunge depths means 25mm (using 30mm, my bit bumped the outside of the mortise and didn't achieve full depth). I don't see this plunge-depth limit as a problem since I don't see a case (for joinery) where you'd need a longer tenon of those smaller thicknesses. Now for non-joinery uses, of course you could use it!
The 500 has three widths vs the 700s two widths. The extra widths allow for slop during assembly and is usually used for panel alignment so one 'anchor' pair of Dominos are exact-fit mortises with the rest allowing for some drift during mortising.
I rarely use the wider mortise widths; I did when I first got the 500, but not so much anymore. A single wider setting is about all I'd require in a machine and both have it.
The height refers to the fence distance to the center of the mortise. Nominally, the 500 should be able to get to 5mm from the center although some fences require a bit of filing to do that (mine for example, but mine is an original first-gen fence). If you have a problem lowering your 500 fence to 5mm, give this article a read.
A limitation of the 700 is that you can't get that close to the center. With a 10mm minimum distance, you can only center the mortise on 20mm or thicker stock. Personally, I've never had a problem putting a mortise slightly off-center and actually encourage that for easier assemblies. However, the 10mm distance means 3/4" ply is about the minimum you can use even with an off-center mortise.
The Seneca DomiShims (or even your own shims) can apply a certain offset to the fence making that 10mm setting be, well, 0mm if you had a 10mm thick plate. The DomiShim for 1/2" stock is 9mm thick so you can technically set the fence to 1mm from the center, but then you'd ram your bit into it. But the point is it effectively gets you a lot of range on the fence height. Granted, installing the DomiShim isn't the fastest thing and, currently, the final version of the shim and fence height gauge are in some flux. Ultimately, you'll be able to use it or some shop-made shim to center your mortises on thinner project stock.
I find the 500 a comfortable size and easily maneuvered. Some people find the top-mounted button awkward, but I've never had an issue with it; use your index finger :)
The 700, while ergonomic in use, is heavy. It begs two-handed use whereas I do a lot of work with the 500 one-handed freeing up the other hand to better hold the stock.
On large projects, people will often go for the thickest tenon that fits the work. Sometimes, though, two smaller tenons are ultimately stronger and much more twist resistant than one larger thicker tenon. Consider that a pair of 8mm Dominos have nearly double the long-grain glue surface as an equivalent length 14mm Domino. Further, two 8mm tenons stacked (with some stock between them) is enormously more twist resistant than a single 14mm tenon; you may not have room for multiple 14mm tenons.
So, which one?
Prior to the Seneca accessories, there wasn't as big of a fuzzy grey region as there is now.
The 500 can do the vast majority of my projects; it does medium to large tables, small boxes, panels, face-frame joinery and attachment. The 700 is now capable, with the bit adapter, to do all those same projects plus those that need the larger tenons; for example, the thick and long tenons mortised through the table top into the legs of the No Comment #2 (Tim Burton Table) build. I often hear that outdoor gates or doors are a prime candidate for the 700 although I'll go back to the discussion that two stacked tenons can be better than one thicker one.
The 500 is still cheaper than the 700 plus some people moving to a 700 are selling off their 500 so you can get a great deal. If 99% of your projects could be handled with the 500, it might be a better investment; odds are that 1% project can still be done with the 500, but maybe with a bit more thought to achieve what can be more easily done with a 700 in that project. If you occasionally do larger projects, the 700 with the bit adapter (or reground 6mm bit) might be a more versatile tool, but at a higher cost for the machine plus the Seneca accessories mentioned.