First of all, I'd say that Merge smaller then other 2. You can check it on http://pokerscout.com/ Also you can read reviews and feedback about any network on this site.There are dozen rooms in each network. I recommend to play on leading rooms (like Carbon for Merge, Poker Heaven for Boss etc). Dont put your money in new noname room  just because they offer higher rewards. I'm not sure about fees, but you could find conditions for each room on their websites. Most rooms dont charge commissions for cashouts.

Carbon, I believe, doesn't chart any fees whatsoever.  I've played some games there, it's soft, I find the half-two thirds-three fourths and pot buttons to be helpful.  Didn't take over 5 minutes to get a game at $33-55 levels.  I saw some higher games running too.I don't know much about entraction or boss.Don't pay attention to the pokerscout reviews though, there aren't enough of them to really have a good sample (means they are ripe for shills) and most importantly, what makes a room good for cash and MTT players (most people in the poker world) does not make it good for heads up sngs.You may also find rooms that offer limited STT-18+ man sngs but have fine husng action.We all also should know by now (reading 2p2's internet poker forum) that every single room has had player issues at one time or another, and often times it's a result of a player doing something wrong (caught multi accounting, caught chargeback, cheating, running some sort of half assed bot on their PC).  Often these players just write angery, false reviews about the rooms that caught them and took their funds.All in all, stick to asking other heads up players how rooms are or try a few for yourself (even if you just sign up for rakeback, don't deposit and just open up the lobby and look around... at least you're ready for rakeback if you decide to proceed).

You could always check different reviews online before driving yourself in to other poker networks you don't know. Sometimes, we have to be cautious about it.