Good Questions Snowflick.
There is quite a bit of overlap in the kites these days. I imagine you would be happy with any of those 3 choices.
Regarding the Access XT 10m. With the double pulley bridle, and recent design upgrade last winter, the Access has caught up with a lot of the Frenzy's all-around performance. It turns quickly in a small radius and delivers plenty of power for high speed runs or climbing hills. The feeling of the Access compared to a new Frenzy or previous model Frenzy, is more personal preference. The Access has a few less cells, and is a bit more docile than the Frenzy. Main difference is boosting, the Frenzy will fly faster thru the window, developing more energy, and lift you higher off the ground more easily. That said, the Access can be a comfortable kite to learn to jump with, and you'll go plenty high with the 10m XT model.
Summary: The Access XT is an amazing all around wing that will open up many enjoyable sessions. The only reason to go for a different model, is personal preference. Riders with more aggressive riding styles, seem to prefer the Frenzy and Manta, as each model offers slightly faster flying speeds than the next.
The new 2011 Frenzy is the GO-TO choice for Snowkiters as it offers everything the Access has, with a faster wing speed and even tighter turn radius. The new design is truly phenomenal, and I think it overlaps the stability of the Access with the speed of the Manta. The 9m Frenzy will have similar power to the 10m Access.
Based on your rider size, you could safely pick any of the sizes you are looking at. Considering you already have the small sizes covered, I would make the jump to the 11m myself, to take advantage of the lighter wind days. It has a wide range and I've flown it from 8-35mph.
The old adage is go for a 9 or 10 meter for the one kite quiver or go for the 6-8m and 11-12m for the 2 kite quiver.