Here are the top exchanges for USD by 30 day volume:
Of these, I like CampBX and bitfloor the best. Bitfloor has lower fees. 0.4% / -0.1% at bitfloor compared with 0.55% at CampBX.
What are your experiences with bitfloor? Or do you like CampBX better despite higher fees? Or another exchange?
You say you are a newb, so I am wondering if you're really going to be buying enough BTC to make the 0.15% difference in fees worth worrying about? That's 15 cents for every 100 dollars.
I have only traded through Mt. Gox, but when choosing another exchange, I would go with whoever had the most liquidity/best prices. A small difference in bitcoin price will swallow up that 0.15% fee difference in a hurry.
All that said, with the shit I see people going through right now with their money at Bitcoinica, and Gox's withdrawal delays, which suggests to me financial trouble over there, I will probably be doing future trading strictly with trusted parties at #bitcoin-otc.
