I assume you're talking about the Modular Accessory & Strap Accessory System?
http://www.tamrac.com/g_modularaccessory.htm
It really depends on your usage, but from the descriptions and photos I've scanned through the site, there are primary differences:
1. Raincover seems to be non-existent
2. Lens cases opening by zips - it is going to slow you down struggling with it horizontally. That's my experience with Lowepro, thought Tamrac may have it differently designed, but whatever it is, I sure can't tell from the photos.
3. How are you going to carry other things like I do? Notepad, CF card wallet, earplugs, extra batteries, small 50/1.8 etc?
One key thing that is recommended for the ThinkTank system that I must explain, which is, the camera body should not be kept in a pouch while being on the belt. It's not a safety concern/call thing, but more so a real world issue, imagine this - once you take your camera body out, would you want to have one empty pouch taking up space on your belt's real estate? It will definitely hinder you physically - as well as this. Imagine you're in the open, and suddenly it rains. Are you going to be able to move as fast with a camera slung on you, or kept as extra weight on your belt? There may be some who would say, "my camera isn't weather-sealed" - to which I'd suggest, keeping an eye out for rain shelter nearby as you move; for those whose cameras are weather-sealed, then you still have the option of moving slower to shelter at the expense of getting a little wet
Hope this helps!