First of all, don't miss the frame geometry details... the 63cm SuperSix and the 61cm Synapse have the same top tube length; 60cm, which is really the most important measurement. Point being, pay attention, because the "frame size" is just a meaningless number a lot of the time.

Can you test ride them? You definitely don't want to "squeeze yourself" into a frame that isn't ideal for you.

I'm only 6'3", but there are a couple of the issues I would want to check out, particular to big guys:
The Synapse's seatpost looks proprietary (or at least, not round). Tall guys often need a lot of saddle setback, and possibly being stuck with their seatpost might not be ideal, although it has quite a bit of setback already.

Big guys have big feet, and toe overlap with the front tire can be annoying. The 63cm SuperSix might give you a problem there while the 61cm Synapse might not, because even though it's a "smaller" frame (the number, at least), the 63cm SuperSix has a shorter wheelbase than the 61cm Synapse.

Either way, I would want to ride them, or something similar so you could compare numbers.