personally i look at it as 1.7.10 for playing kitpvp, warmup, etc.. and 1.8 for playing factions, survival, big multiplayer servers, etc.. mainly just the typical servers. that's just how i look at it.
At least while using Vape, there are really no differences in how the client functions / operates version-to-version. It is entirely up to you which version to use, so long as Vape supports the version that you are trying to use. Either one is good.