In the Mustang forums most folks feel that an X pipe will give you a few more horsepower, but its in a fairly high RPM range. In normal driving there is very little difference.
In most cases the X pipe also has a higher pitched tone than the H pipe. The H sounds more like a muscle car rumble, while the X sounds more like a highly tuned race engine (if that make sense). Many hotrodders prefer the more classic tone from an H pipe.
For a 302 2v you need headers with small diameter primary pipes to keep low end torque. The 2 1/2 inch exhaust is overkill for the 2 barrel (2 1/4 would be plenty), but it will still work fine. Once you get a good dual plane manifold and get the carb set up right the bigger exhaust should pay off.
You don't mention what vehicle this is in, but make sure your cam choice matches the vehicle weight and how you plan to use it. High lift cams usually work best in relatively light cars with low gears and a high stall torque converter. If you put a high lift cam in a heavy vehicle (pickup truck?) and try to use stock gears and converter, you won't be happy.