Haven't got one and never used swim stroke calculators, although masters session they do make us count manually sometimes. Not a brilliant swimmer so for me I don't think it owuld be much benefit other than to feed the data beast.
Reminders, probably quite useful. With 10-minute reminders when tired it is easy to look at watch occaisionally and then tack a bit more on 10:53, I'll start at that tree which may well be 11:15. The second reminder forces you to go. £150? Nope.
Barometric altitude, only if you want to hang out with ultra ultra runners and maybe some bike freaks. Are you are the top of the hill, no, then you have more hill to climb. Altitude is available on 310XT but GPS accuracy for altitude is not brilliant and barometric is better.
I have 310XT and if it broke I may get the 910 but certainly don;t see enough reason to upgrade.