I didn't dislike Ruby 2.0, but I did like the original incarnation more. Ruby's storyline probably would have played out in the same fashion no matter who was playing her (not going to go into details so as to avoid spoilers), but I preferred Cassidy's performance over Cortese's. Stupid budget cuts!
I saw Ruby 1 as being a bit of a Mary Sue. She's the only good demon in the universe, with a demon-killing knife, who knows how the Colt works. She's a massively overpowered protagonist.
Of course, we know how that was supposed to work out, if not for the writer's strike. Ruby 2 is still competent and still extremely dangerous, but she doesn't know everything there is to know and isn't perfect.