If you do choose an inside, then be sure to bring a nightlight. You can keep it in the cabin or leave it in the bathroom, depending on how much the light will affect your sleep.
You can also tune in the room TV to the shipcam. It will show little to no light for the night time but will brighten as dawn arrives.

Check out cruisecritic.com for lots of tips.

Check out vacationstogo for some cruise deals. Their July 2008 sailings on Celebrity Mercury show starting prices of $799 pp. (I don't know if that's when you're planning to go, just thought your $4100 was high.)

That $799 starting price is per person for an inside room, which is where I got my $1600 figure. The $4100 is for two people in a balcony room.

The idea of getting one balcony room's a nice one, and if my brother and I can swing it it would be nice to upgrade my parents room for them. The night light and ship cam ideas are also great info. Thanks!