Well im building a 302 just because the cost to build it is cheaper and its alot easyer to find used parts for them... plus if you have a tendency to over rev or put it on the bottle, (therby destroying the block or internals) a used 302 is easy to get and you can transfer all of your parts over without any problems. (most of the time)
At the same time though if your going through the trouble of building it right insted of cheap, the 351 would be the better option... cubes are king.