A while back, I got the itch for a Sinn. Having never worn any in person, I narrowed it down to either an 856 or 756 because I like the large 12-3-6-9 numeral design. After a few months keeping an eye on the forums and ebay, I picked up a 756 from someone on ebay (whom I believe is also a member here). My initial reaction was that it's quite chunky...which it is - 40mm case diameter x 14mm thick. It's like a tuna can! Despite this, I actually find it's quite comfortable, and the thickness is something I got used to very quickly. My Omega Speedmaster is also 14mm, but the Sinn wears a little thicker since the case is 14mm from top to bottom, whereas the Omega has a domed crystal and tapered caseback, with the actual case being thinner. I do wish it had a seconds hand, but I could always run the chrono for a while if I felt like I needed something moving to look at All in all, it's a great watch, and I don't regret buying it one bit.
But I still was interested in an 856, and I just took delivery of one earlier today, which I purchased from another member here. My initial impression is that it's not chunky - 40mm x 10mm, a considerable 4mm thinner than the 756, and definitely proportioned more 'normally' than the 756. Although now that I'm accustomed to the 756, I find both to wear comfortably. Had I gotten the 856 first, I may not like how the 756 wears. The dimensions of the 856 are right in my sweet spot for a wrist my size, IMO - 6.5". The 856 seems to be right on par, quality-wise, with the 756, and it's my first UTC/GMT, so I'm looking forward to wearing it while traveling.
Aside from the thickness, and the obvious chronograph vs. UTC complications, they're nearly identical watches on the outside. Same case diameter, same general dial/hands design, same bracelet, etc. My one gripe about them is the bracelet. It's pretty thin and chintzy, and it seems especially thin on the pudgy 756. I don't think it'll fall apart or anything, but I don't think the quality of the bracelet matches the quality of the watch. Other than that though, they're both excellent watches. My plan was to sell the 756 once I got the 856, but now I'm having second thoughts...although I'll probably come to my senses since I think they're too similar to keep both.
Both Sinns, side by side
From the side, you can definitely see the difference in thickness, and how thin the bracelets look.
The 756 on the wrist - for reference, my wrists are 6.5"
The 856 on the wrist: