second stages are all essentially identical, only difference is the chrome trim on the covers and what not, the aesthetic differences in the pictures are literally the only difference.
First stages are where the differences are, but all are equally well sealed using the same technology.
XTX50 can come with the following
DST-Turret, most commonly copied first stage on the market, Hog D1, Dive Rite RG3000, Hollis DC1, bunch of others. My preferred backgas style regulator. 5th port makes backmount singles and doubles hose routing easier, turret is an added perk for backmount, but really great for sidemount
DS4-similar to DST but no rotating turret. This is the one you usually see backmount doubles guys using, stupidly reliable regulator, but I prefer DST because it is more versatile for backgas. DS4's are great for stage bottles though.
Tek3-basically a DS4 but 3lp ports and 1hp port on one flat face instead of 2/1 on each side. Doubles only, work really well for that especially if the 5th port conversion can be put on the bottom of them, but very expensive and not worth the upgrade cost over the DS4 or DST's
XTX100
FST-only option, don't see these very often, good first stage, but no advantages over DS4
XTX200
FSR-similar design to the FST but a bit different, has a removable hard seat crown which is nice I guess but it can only get messed up with bad regulator repair. Since Apeks has lifetime warranty with the PFL program, the others would just be replaced in that incidence.
Look long and hard at your future dive endeavors, if you see sidemount or backmount doubles in your future at all, it is cheapest to buy two sets of XTX50 combo's and have the extra first stage available to you.
Dive Right in Scuba will have the best prices more than likely.
XTX50 - Dive Right in Scuba
Edit: due to response below I did forget you can still get ATX40/US4 series which are good for deco bottles and what not, and do not have the ability to reverse and they don't have the wide exhaust ports. The XTX series have a user changeable exhaust port which allows you to use "normal" T's and "wide" T's which do actually help for bubble trails