It all depends on which night you go.
Let's assume it's on a weekend night though (note that on weeknights there are less locals in attendance and things are not as rowdy).
I would rather go to the Boardwalk. The reason is that, I like to have a casual drink with my wife but Im not into the "lets get drunk and party" atmosphere.
City Walk by far has the best atmosphere and has the best restaraunts (IMO). You can sit at Mararitaville and look over the water into IOA. However, like someone else said, at City Walk it's a very young, and ready to get boozed crowd. At some points I felt like I was IN the dance club when I was OUTSIDE walking down the sidewalk. At some points I could even see how things could EASILY get out of hand over there. I saw VERY little security.
Pleasure Island. This area is much more family friendly, but on the weekends the locals come out in full force and they're not all Disney families. While you may be there on vacation with your kids, you need to note that some of the 21 year old locals are not on vacation and don't care about Disney and are there to party. On the weekends there we've felt very uncomfortable at night.
Example: In March we were there and actually left early one night (from just walking around) because there were people screaming and running down DTD drunk and there were people publicly making out all over the place. I couldn't believe that there was that whole attitude there. It sort of disappointed me that it could possibly be Disney.
The Boardwalk, while not being very big and not having any many things, has the PERFECT atmosphere I like. Epcot just across the water, the beautiful atmosphere on the lake. You still have restaraunts.
It has that romantic appeal and family appeal together. I love it.
It just depends on what you are looking for. If you are a partier though, honestly as nice as PI may look to you, overall Citywalk is a cooler place.