HW Question

[Tam_Hawkins]

If and only if you are already sure of buying havensward somewhere along the way then yes you should buy it (I mean why not?).
Heavensward (or more like lvl51-60) changes BLM quit a bit so you should probably look up if the lvl 60 playstyle is still your thing, but otherwise the games are very similiar so if you liked ARR you should also enjoy HW.

*Edit: YOur concern of blowing away experience: during my time of leveling up right after HW release there were times when i needed to grind dungeons to keep up with the level, but it was never much normally 2-3 Dungeonruns would be enough to get to the next level. Now they have added Beasttribes (daily quests) and some extra side quest that should help you with leveling up so I really don't think getting enough exp should be a concern.