both will work in PvE, though neither will accel at clearing heroics. i enjoy playing my 4 pally DK team but in my early days of boxing i played priests and truly had a blast with them. they're a much less common team - they've lost some luster with repeated nerfs to the group healing of shadow (it used to be phenominally OP and cool as hell) but you could effectively play them all healing spec and still do efficient and good damage now that cata buffed the hell out of healer damage. i'd say if you're thinking about doing priests then do it, you'll have more fun knowning you're playing a group that is yet to be the FOTM.