I had my TT bike set up 1x11 with 50 x 11-28 for a flat 70.3. I could spin out the 50/11 on flats with a tailwind at race pace. I'd go with the 50 to get you a little more in the middle of the cassette more often and have 50/10 as an "overdrive". I never went into the 50/28, I think i might have went into the 50/22 when i was on a false flat with a nasty headwind. But it's nice to have some bailout gears, especially in training when you are trying to just have a recovery ride.