I've used Item and 80/20 as well and have found that i like the
Frame-World extrusions the best. They are a good value, and have a nice clean look to them (no grooving, available in several anodized colors). The T-Bolts for the frame world stuff are also very strong as well as inexpensive.
The structural strength of the various extrusions are very similar, so it seems to come down to price and availability of accessories.
-Q