857 is a GMT (ETA 2893-2).I was in the same place as you a few weeks back. My criteria was a bit different, which I assume you know by reading the thread I posted. I am not sure what you like to do when you say 'hard use'. If you plan to use them normally, either one holds up well, much better than other watches....tegimented ones better than non-teg. In case you are looking for a watch to double as an occasional hammer - U1 is the way to go. In all seriousness....give some thought to what you currently have, what you are looking for, and where you will be wearing this watch. U1 to me is very unique and has an understated quality as opposed to one that screams for attention. 857 is similar too, but not as subdued. If you do not want a chrono, go with U1. If a chrono is must, 857 it is. I know everyone says this but its true - you can't go wrong with either!!!
According to Sinn the U1 does have some magnetic field protection. " This special steel is also amagnetic which means that is does not have magnetic properties and cannot itself become magnetized. The watch movement itself is protected from magnetic fields up to 4,800 A/m."No first hand experience in these models, and it seems a tough choice:
U1 sub.steel <> 857 Tegimented steel
U1 solid bezel <> 857 aluminium insert
U1 100 bar resistant <> 857 up to 20 bar
U1 no magnetic field protection <> 857 up to 80,000 A/m
I'd say they both survive everything you do, and even more. Choose the one that pleases your eye the most, a modern diver or an instrument pilot. Or follow the usual WUS answer; "get them both". :-d
Good luck in decision making!