The 302 (5.0L) is a very competent engine. Lots of parts for it, and the Small Block of choice for most street Mustangs (1979-1995) and cruisers (1964.5-1973).
The 351 (5.7L) does come in 2 flavours. THe Windsor(351W) and Cleaveland(351C).
The Windsor was built in Windsor, Canada. It is a 90* V-8 small block.
The Cleaveland was built in Cleaveland, Ohio. It is also a 90* V-8 small block.
The true differences are that the 351W shares heads with the 289 and the 302. The 351C has Oval head (similar to a Hemi). This head/intake setup provides less restrictive valving and intake runner path.
The Cleaveland is the "more bang for the buck" stock motor. I don't know about modified 351's, however the Cleaveland was the better motor originally.
There is a variety of aftermarket parts for both, however if you purchase one, make sure you get the right parts as 351C parts will NOT fit 351W motors and vice versa.