Bovada has the superior software and the superior hand history accessibility. The hand replayer's functionality is great, and you can view opponents' hole cards, when they go to showdown. Also, since Bovada is anonymous, players are allowed, after an allotted number of days, to see every opponents' hole cards in the hand history files. This feature is invaluable when reviewing hand histories and evaluating what strategies were effective. And, though many people may not be looking for it, Bovada is the only site that has consistent Fixed Limit Hold'em action.
If your bankroll is on the smaller side I would recommend ACR as Cardschat holds two weekly freerolls there for members in the FreeRollClub. (ACR,BCP)
Sites on the Winning Poker Network (ACR, BCP, etc.) also have a near-constant availability of their "On-Demand" freerolls. As soon as one starts, there is late registration for thirty minutes, and then the next one starts registering. Another benefit to players with a smaller bankroll is that WPN offers smaller stakes than Bovada, both in cash and MTT offerings, though they both offer SNGs that start at around the same buy-in.
I haven't played at Bovada in a long time so I don't know anything about there VIP/Rakeback promos so I can't tell you which is better in that department. Though I would assume it would be ACR.
It is WPN. They offer actual rakeback, if players sign up through an affiliate. Bovada has a points system that allows players to convert points into tournament buy-ins, but not actual cash. For me, this makes no difference, because both points and rakeback simply allow me to play more poker. Because of that, I use Bovada to focus on tournaments, and I use WPN to focus on cash games. That being said, WPN offers both rakeback as well as a points system, though I am not familiar with how their points system works. They seem to have numerous promotions going on at once. Full descriptions of ACR promotions can be found
here, and I am actually going to be looking through that page, myself.