I've never been inside Rock Bottom, so this review only applies to Sing Sing.
This is a place to have a fun night out, as others have pointed out it is a larger venue that its local piano bar competitor "Charlie Murdoch's" down in the south side.
In terms of a bar, its nice, large open, stage is visible from pretty much everyone, and the staff is entertaining. The bar offerings and prices were relatively pricey, not the "are you freaking nuts this is Pittsburgh not Manahattan" variety, but you do pay a premium, think of it along the same prices you get at a comedy club.
Now in terms of service, while not bad, its nothing to write home about.
This is one area where Charlie Murdoch's, at least in my visits, wins hands down.
The entertainment at both places is comparable, and on I'd like to say on a busy night that Sing Sing parking would be less painful than the southside, but on a busy night, I gotta tell you I don't think being in the waterfront give Sing Sing a benefit I've had to park pretty far away down there at times.
All in all this is a fun place and a great places for a group of friends to get together to celebrate a special occassion etc.