I've used both brands, colors and clearcoats from each. I'd say the biggest difference is the spraying/atomization of the paint out of the nozzles. Testors is very heavy-handed, requiring you move quickly past the body to avoid runs (in my experience). Tamiya sprays much better, IMO. However, I think Tamiya is thinner and can pull away from corners more readily than the thicker Testors.
Best-case scenario, use either one for color and clear with Tamiya for a smoother finish to polish.
I've used the Tamiya primers and they are nice and fine, less "grainy" than the 1:1 automotive stuff. But I like that too, so...