I got the Summit one its the same as the trick flow and it was in stock, I did notice the lip hangs lower than than diff by about 1/2 inch but sealed up real nice so dont think this is a problem at all.
I know the FMS is definately a copy of the T/A. In fact T/A may even produce it for Ford. Nice design though. I went with the T/A also mostly for looks. Without the stock gas tank you can really see it from behind the car.
Its worth the money just to help when you drain the diff. Taking out one set screw is alot easier than taking the whole cover off. Not to mention you dont have to worry about stripped or broken bolts as much.