The balconies on GP vary: some are fully covered, some are uncovered and some are partial. The partial ones I've seen are actually double sized: 1/2 covered and 1/2 uncovered. There are some experts around here who can tell you which are on which decks.
GP was pretty dead last December (first week of December). However, we had a big enough group that had met on cruise critic that it was still a week long party. Other than Skywalkers (dance club) though, the ship was dead by 10:30. We're on again 12/3/2005, and we can at least guarantee that somebody will be in Skywalkers past 11pm. 
I've never been on Conquest, but the decor on GP was a bit classier than on Elation (which itself is supposed to be the more understated ship in it's class, but not the same class as Conquest). I would bet GP is a bit nicer that Conquest, though that is mostly a matter of taste. My pictures are here (http://community.webshots.com/user/mikesteg) though not too many interior shots. Lots of others on webshots though.
Price wise, I think you'll find them very much in line (GP and Conquest). Elation is cheaper for insides, but if you go for a balcony again GP should be cheaper since she has so many more than Elation.
GP has booze by room service but not in a liquor shop, and it's a very limited selection. It is a bit more economical since there is no fee to take the booze back to your cabin, but still about double street price. You can also "send a gift" to yourself to be in the cabin, from the Princess website.
