I've got both and use both on a regular basis in a commercial workshop making furniture both large and small.
If I could only have one then I'd have the 500. If at all possible I'll always use a Domino with the sole plate flat down on a flat reference surface, in this orientation the ergonomics of the 700 wins. But there are times when that's just not possible, often in chair making for example the Domino is waggling around in space, and in those circumstances the 500 is much more manageable. So unless I was only making big beefy external doors I'd have to say the 500 is more versatile for my applications. Your applications of course may be different.
But the original Domino 500 is getting a little long in the tooth, so it's also possible an upgrade is on the way incorporating some of the features of the 700? Just a thought.
Another thing to consider, do you need a Domino at all? I realise they're all the rage, but are they right for you? It's worth remembering that with a Domino the squareness of the components completely determines the squareness of the final frame or the carcass. So unless you have the skill or equipment to consistently produce really square, straight and true components then a Domino may just embarrass you.
The other negative about a Domino is the "tyranny" of the available Domino tenon sizes, if you design your furniture to fit the standard Domino dimensions all well and good, but if not there are many times in furniture making where you're faced with making a not-quite-optimum Domino joint (for example where a haunch would be just the ticket, or the dimensions work against you) or making an optimum joint by other means. After all, a Domino doesn't make better joints, it just makes good joints quickly but only in flawless components.
I'd recommend that anyone thinking about getting a Domino considers what their real objectives are,
-You're a professional or advanced hobbyist with an excellent quality planer/thicknesser and dimension saw, both of which you're thoroughly experienced in using and maintaining. You want something that'll speed up making bread and butter joints so you can focus on the more interesting stuff. Yes, a Domino's for you.
-You're a well healed hobbyist who enjoys using the latest kit. Nothing wrong with that, it's your money so you spend it as you like. Yes, a Domino's for you too.
-You're a beginning hobbyist with mediocre quality basic workshop equipment that you haven't fully mastered. Your Mortice & Tenon joints are a bit sloppy and you hope a Domino will improve matters. No it won't, until you sort out your hand tool skills or your basic workshop equipment a Domino won't improve your jointing, it'll just add to your frustrations.
Good luck!