this topic has been talked about a lot. Take a look at the "Rebuilding Stock Seat" topic down a few. LOOk at some of the old topics, you will pick up a lot. As to which one is best I think you will be happy with Mustang, Ultimate, Corbin, or the rebuilt one. Rebuilt custom is cheaper.
I also think you need to consider part of what you do is just toughen up a little. One problem is just sitting in the same position long periods of time and using muscles you don't normaly use. Also think about Lambswool, or a gel pad. Maybe just ride for a while and make up your mind later if you are new to it.
I once went to Walmart, bought a lambswool mit for washing cars, took the hem out of the thumb side and sat on it. For $11 a big improvement. Look at gel pads on Ebay