<~~~ We don't have kids. We're in our 30s now, and have gone 2 times together (26 and then at 32). My first DCL was during HS. Unless you are going single or *need* a casino, I'd always recommend DCL. It's not exactly a place to mingle with other singles (unless you are a Teen, then it's a lot of fun), so going with a GF is a great idea.
You eat dinner every night with the same people, your table mates and servers rotate with you. (This is a good thing, don't flake out and skip dinner). Disney does a great job of seating you with other couples of the same age so you make friends at dinner the first night and can then hang out at the pool and bars together. The nightlife isn't as crowded as the videos make it appear, but if you don't mind mostly dancing/drinking with your 4-6 new friends, its a fun time. Plus, the excursions can be as kid-free as you'd like.
The adult's only pool/beach areas and bars/clubs make it so that you almost never see kids. Also, book the later dinner time (8pm), that way you see the show first, then eat second. The families with little kids eat dinner first and then put the kids to bed.
The bars close at Midnight...Let me tell you, after sipping the Drink of the Day (all day), being in the sun, having a cocktail before the nightly show, having wine or liquor at dinner and then drinking at the bars/clubs until Midnight...IT'S TIME FOR BED. Trust me, and if you want to keep the partying going, get the beer or whatever package in your stateroom (eh, be kind to your sleepy neighbors just hang out on Top Deck). The boat doesn't ever close.
Cost...the price of drinks is not high at all (not compared to NY prices). The "Drink of the Day" is $5.46. You can also get a beer mug with refills, and there are wine or beer packages for your room/dinner. This isn't Carnival where you are expecting drunk college kids at the pool. That being said, you can easily be drunk, but in a Disney friendly way haha.
Skyline Bar on the Fantasy: ~$12 (cheaper than in NY, and you keep the glow cube lol)