If you're on the same 50/34 and aren't really using the 34T, then go for the 11-28. For most people and most roads, a 34-28 bottom gear is enough. If you ride for a bit and find it's not low enough, then you can always replace the cassette easily enough.
You can get an 11-30T cassette in Shimano 11-speed if that's a better option for you (my preference, just for that extra gear if I hit an unexpected steep pinch or am weak/tired).