Potentially it would be better to wait? Most people think that when they release the new xpac the content from HoT will then become part of the F2P bit so new players will only ever have to buy one new expansion. But I don't think they have said for definite sure that is how it works yet. If nothing else though I'd try to pay attention to the Living story release schedule because if you login in once during each active chapter it'll be unlocked for you already when you get HoT. So, for example, right now the fourth chapter of Living Season 3 is active and if you login before the fifth one is released the fourth will unlock for you even though you don't have HoT yet (you just wouldn't be able to play it until you got HoT).