Granted I've always leveled as feral, I've had my good deal of exposure to soloing/questing as moonkin.

In terms of damage and the ability to destroy mobs, both feral and moonkin get the job done. In terms of survivability, feral wins hands down. If you're from a PvP server, running around in kitty with your Humanoid tracker up is a godsend. Being able to have improved stealth abilities and passive run speed helps not only with the PvP aspect but if you run into stuff that you just want to avoid in PvE. Every new expansion, there's always a sizable crowd on my server that is dedicated to hardcore "wipe out the ppl in the starter zones" rather than leveling, so I prefer to not let them get the drop on me. ;D

If you're thinking of leveling before the expansion, I'd say the starter zones are fairly empty, and PvP really isn't that big of an issue until you get to later zones where 80's start farming herbs/ore or dailies like mad. Either is fine right now, you just have to be a little more careful as boomkin (or get into the habit of eating the food that lets you track Humanoids).