Buying a triathlon wetsuit/speedsuit...?

I'm relatively new to the Ironman circuit and running my first 70.3 next season and in the future I want to run multiple full Ironman races and ultimately qualify for Kona (I've been told by some Ironman veterans that my first few Oly and sprint distance races were top quality and in the top 25% of the field; I'm also looking to set a new marathon PR in May at the OKC Marathon). I'm looking at a wetsuit/speedsuit for two races I'm looking to enter in the 2011 season and I had some questions about the suit itself and the sizing. First of all, is it typical to wear tri gear under the suit or to keep the suit on and go straight to the bike and run as to save time in transition? Would wearing tri gear under the suit effect the size or fitting of the suit? Also, what is the difference between a trisuit/race suit and a wetsuit/speedsuit? I've estimated my chest size to be about 44" and waist line at about 34" and height/weight is 6', #180 so I think I may need to look at getting a med/large or large. As for brands I was looking at Xterra or Orca but open to suggestions. (looking mainly at a long leg speedsuit like the Xterra Velocity and the Volt since I don't have $300-$600 to spend at the moment).