Both are perfectly adequate for healing 5 mans. I would lean towards Disc though because I just find it more fun. Up until Cata dungeons also the healing is extremely simple, as an atonement spec lazily using holyfire/smite will get you through no problem, without atonement a single PW:S will most likely get you through most pulls.
Once you hit Cata level dungeons you have to start playing a bit smarter, atonement may not be so viable for pugs as healing ranged can become a bit of an issue if they're spread out. I found playing without atonement to be much more rewarding and flexible, as long as you get the most out of rapture and use your cooldowns wisely then mana shouldn't be such an issue, even without gear.
