Ultimately, both the trinket and the belt are significant upgrades for you and it's hard to tell which one would benefit you more. Probably the trinket, since mana will be an issue for you atm.
However, you might want to consider that there is only one resto belt dropping in ToT, as opposed to 4 healer trinkets. While not all of those trinkets are equally good for us, they're still big upgrades over what you have.
If you're gearing this for the long term, I'd consider waiting until you're almost capped to see if you can get more to drop from ToT LFR so that the VP doesn't go to waste. Or you could buy one, do an LFR, then resell it, until you're almost capped on VP and have to make the choice. If it's short term and you want to raid some non-LFR content soon, I'd go for the trinket.