SY77 all the way! Unless you really want the 'legend' that is a DX7 on your synth stand! (and no problem with that). They are both great synths (The MK2, S DX7 more than the Mk1 IMO) and the SY77/99.
Soundwise? The SY will do stuff no DX7 can (not even talking PCM stuff here which I never use - always pure AFM mode). But they both do the 'DX7' sound just fine. Some say the SY is impossible to get that feel from but it's not true, and no you can't convert patches easily you have to work at it - but it's like any synth > synth you'd expect that! doesn't mean the SY can't do it (if you programmed the SY from scratch you'd come up with just as many DX7-ish tones as on a DX7! if that was your goal)
Hope I'm ok to post a link to the GS forum here? if not mods please let me know and I'll remove it - i posted this last month a comparision of a couple of custom patches on the DX7s I converted to the SY77 - they are not 100% identical but hopefully as you can hear close enough, in fact the SY ones sound better in some cases. http://www.gearslutz.com/board/8177432-post236.html
If you don't want all the fancy extras or the look of the SY77 then by all means get a DX7s/MK2 as they are nice synths. Unison poly is lovely sounding on any non MK1 (Mk1 doesn't have that!) on the SY you can use 2/4 elements and detune them for a similar (though not exact but no worse) effect. One omission is lack of 3 element POLY FM only 1 or 2 FM layers. If you have 4 it's mono. and there is no 3. Why would you want 3 layers poly? to get closer to the DX7 unison sound. If you are talking mono sounds then the 4 AFM mono mode is a monster! Not only can it make a stab at analogesque stuff, many times I prefer it (I used to have an SH-101 for slippery lead lines with portamento - but the SY did what I wanted with a different/unique edge more often so I got rid of the SH-101s!)
As for 99 v 77 - as i only use FM and 50% of the time NO onboard effects (nor would I ever use the sample ram thing) I'm more than happy with the 77 even though the 99 is obviously the 'better' of the two. As a whole unit though, I prefer the size shape - even the name ;P of the 77 (a hint at it's predecessor) and the 77 itself is quite heavy/bulky as it is - built like a tank - a 99 must be beastly! too big for my needs. 77 is fine for me. If I didn't have that a DX7 mk2 would be fine too - lovely synths once you learn how to truly work FM!
And yes in general the 77 goes for less than the DX7 Mk2/S because it's less well known, less sought after, no legendary name (I am one of thousands of 80s synth kids who dreamed of owning a DX7 back then - from seeing it on TV so much if nothing it else) and because it's a nice compact - purposeful synth. Some people abhor any markings of 'workstation' - I too would prefer the '77 didn't have a sequencer (or PCM for that matter!) - so while it's a bit of a 'secret' that the 77 is much more powerful than the DX7 they are respected among proper synth heads. The DX7 is appreciated by anyone who doesn't hate them! Bigger market basically, more bids, higher prices - classic piece of gear.
I got all my synths pretty cheaply anyway, I actually paid the same for my 77 as I did for my DX7 II-D
and I paid a bit less for my 'bargain' DX7s
(which I preferred in some ways to the II-D odly enough!)
I think really it comes down to if you will appreciate the extra features of the 77 more than the 'kudos' of the DX7 logo. I can see a case for both and the head goes with the SY77 while the heart with the DX7 for all the obvious reasons. Only you can decide. Oh and you are more likely to find a DX7 for sale (glad you are focussed on MK2s but don't forget the DX7S - great synth! can't do layers but does have unison and backlight, cleaner outputs of the true mk2s - I say this as you may pass one up otherwise and if you are searching...)
Here's a pic of my SY77 with my DX7s above it (which is now sold)