What boots you get are totally dependent on your foot structure, skiing style and weight.
A quick overview of Atomic's boots...
The B Tech are the widest boots on the market with a 104mm last. They are Best for someone with either you guessed it, big, wide feet, or someone who dosent ski all that much and is more concerned with comfort than perfomance.
The M Tech (or T Tech before 05/06) are a medium volume boot with a 102 mm last (IE C width feet). These boots are a safe bet cause they are an easy fit for most people. They are a little more narrow thruogh the heel area than most other brands, so they hold your foot down great.
The race tech or plug boots come in two varriations, the TI, which is an ultra narrow 95mm last, and the CS, which is a more forgiving 98mm last. These boots are made to be customized with thick plastic to grind and punch out and super thin liners.
Speaking of liners, they thin out as you work your way down the line, with the B tech having the thickest, then M tech, then CS, then TI. The thicker they are, the more plush, obviously, but also the more they will pack out.
Flex indicators run from 50 to 11 in the B and M tech, and 11 to 150 in the race tech. This is a matter of weight and skiing style, and its tough to gauge w/o a bootfitter or good salesperson.
For example, I ski in the Race Tech TI 130, which most people would say is waaaay too stiff for me based on my weight and the fact that I freeski and don't race. But its really the shiznit with a few modifications. You need to try them on, flex them, and take into account that the store is also much warmer than where you will be skiing them...
Oh, also, im not positive about the AFT boot since it was a little before my time. I think its a B tech fit, but Im sure Rex would know, ask him.
hope my little essay helps...