Single target, hunters are slightly better than spriests. Throw in another target, it's a different story. But that is the same with any multi dotter. Although with the srs buff hunters aren't miles behind on multi target either.
It depends on your guild really. Go with what's needed most. Spriests scale really slowly though so keep that in mind. At lower gear levels you'll be struggling. But they balance out in the end. Hunters provide slightly better utility in current raids. And can bring pretty much any missing buff/debuff. Spriests can off heal and even respec to a full healer if needed. So again, it's really up to what your raid make-up is and what your guild needs more.
I didn't know the SP scales slowly.. I mean I'm kinda tired of being medium table dps, always dk, locks and mages on the top, and Im pretty sure is not for the rotation or anything, At least on my guild, I'm pretty sure I can beat them if we were on same conditions fight, although my Rl keeps telling me that when I get a proper weapon Ill be on top of the metters again (I still have a blue weapon... no luck at all in lei shi, i go every week and nothing, and same for the will....) and almost everybody on my guild has a decent wep... so I though maybe rerolling to SP would change the history...
If you want more utility definitely roll with shadow priest. Hunter has a more solo fun factor and is slightly better single target dps...but spriest more than makes up for that with utility/versatility.