At least the base game, you're not really losing much if you buy the PS3 version now and later get the PC version (or if you just buy both versions now). They're currently running about $16 each (based on Amazon's prices - $16.00 PC and $16.12 PS3 - check around, maybe someplace else has them for even less) and you get a month of subscription time with each of them. So it's a platform's access plus a month of subscription time for only about a dollar more than just the month of subscription time alone. Get both and you'll have two months of access and be able to play from either platform.

When it comes to adding expansions, those don't come with another month of subscription time, so duplicating those purchases would be an additional expense. Buy the version for whatever platform you're primarily using at the time when you reach that expansion's content. So that may turn into something like buying Heavensward and the 4.0 expansion on PS3 and the 5.0 expansion on PC if a few years from now when 5.0 comes out you have a better PC than you do now.