Either of those cats are good, they are both high flow and stainless steel.
3 inch exhaust is ok if you plan on extreme horsepower, otherwise a 2.5" exhaust is good, and you still maintain some low end torque.
The best possible exhaust setup for a "V" engine is to have a separate pipe for each header going out as duals to the back. With a pipe connecting them making an "H", where the top of the H is where the headers go into the exhaust, and the bottom of the H is the exhaust tips. The pipe in the middle equalizes pressure and is known as the best way to increase horsepower in an exhaust system of a "V" engine. Obviously you could not do this with a 4-cylinder.
The only concern with H pipes is ground clearance.
1990 Ford Probe LX 3.0L V6 :s6:
SEE IT HERE:
1989 Mazda RX-7 Turbo II 13B-T "FC3S"
Turbo RWD Rotary Powa!