Thunder,
I have used all 3. To me, I would give the 910 a 5, the 920 a 8 and the 935 a 9.
910 - it works. It does what you need in that it records swim, bikes and runs. It is big, the buttons are hard to press, it has a small screen, etc. but it works.
920 - better screen, easier buttons, a bit smaller, good battery, connects with your phone, the downside that is unique to me is that I put my mount just behind where I fill my BTA hydration and since the buttons were on the top and bottom of the 920, they would often get pressed accidentally during races and when you in multisport, that completely screws everything up.
935 - great batter, great screen, much smaller, all the benefits of the 920 plus integration with SRAM etap among other things. I wouldn't say there is anything earth shatteringly better about it than the 920, but it is better. The optical HR is a mehh for me as it is not, nor is any optical HR super reliable. Oh one other thing I like is that you can make it so that on swims, including in multisport, you can lock the buttons so you have to press them twice to advance to T1. I have had people bump me on swims when using a 920 and it goes to T1 some portion of the way through my swim. One other thing is that the 935 will continue to be updated for awhile by Garmin. The 910 and 920 only for bug fixes.
2018 Races:
IM Santa Rosa,
Vineman Monte Rio,
Lake Tahoe 70.3