Welcome to CC!
First off, some sites are linked together but I don't think you really understand it. Cake and Lock poker are both on the Revolution software. The games they offer are about the same, but it is not the same players. I don't think you can actually load up Cake and Lock at the same time, and its not like if you play in a tournament on Cake there will be people at your table that is playing on Lock.
Carbon, Blackchip, and RPM poker is all on the Merge software (and more.) They all offer close to the same games but like I said earlier, its not the same players playing against each other.
Also, don't put too much into the people with 50 post or less that complain about a poker site
. Now-a-days, everybody that finds a problem with a site or a issue that there having will take it to the poker message boards.
If I was picking where to play, I would choose carbon. I personally like the Merge software more than Revolution. Carbon also has the most traffic out of the Merge skins (maybe the most out of all US sites?) I did withdraw with Carbon last summer. It was a annoying process...but once my information was cleared it took 3 weeks - a month to get my check, and that's what they said it would be. My only compliant about Carbon is there support team...you have to go through email, and they take 10-24 hours to get back to you.
has great support team. They have live chat, so any problem you have gets fixed in just a few minutes. Also, I had a very easy time getting money out. I did a moneygram on Betonline about 3 months ago, and got my money that day. There tournament selection though is awful. Just nothing to play. If you was a cash game player then maybe...but as for tournament they really don't offer much.
I have played on Cake some, really like there tournaments. There support team is solid, you have to go though email but they usually help you with-in a hour. I haven't did a withdraw on there.
Hope this helps!