Both roles are important, so you should find no real problem finding a spot in raids.
I think in this case it's more up to:
1) Which one you like playing more
2) The needs of YOUR guild
Every guild has different needs, though in some guilds (like mine) tanks may be selected way more carefully, in which case Resto would give you an easier entry into guild runs.
So, main roll Healer, OS roll Tank - There are only two tanks and possible one more guy rolling for tank gear as OS and you’re 4th, meaning you’ll gear that OS very quickly. Once you start raiding HC content as a healer, run normal as a tank and there you go.