The important question is at what lift, and what flow rate? The Magnums should flow better with no work than the 302s with some porting $$ incurred. The 302s should have better low lift velocity and flow rate, but you'll need to spend money to keep the numbers climbing thru the whole lift range, especially when larger valves are installed. Magnums in cleaned up form will make up to about 450hp pretty easy on medium sized cams. 302s will need a bit of work, and much more cam to make numbers near that.