if they're the same price and similar condition, always go for the bigger motor. with one caveat.....if one is a roller cam motor, go for the roller motor....if you can find a roller cam LA 318 (M bodies 85-89, trucks 85-91) or roller cam 360 (trucks (87-91) that can get by with a rering, go for those. the roller 318s have decent closed chamber heads (302's) the 360's have better 308 heads. the big bonus is the roller cam, which if in good shape can be reground and reused. I'd also take a magnum 318 or 360 if they are similar price for the better heads and roller cam. see my response back on page 2, I did both, and my reringed '85 short block, magnum headed 318 with a reground roller cam made as much power as my reringed flat tappet 360 with the same intake, carb, and headers, but did so while also getting SIGNIFICANTLY better mileage (IIRC, since it's been 20 years, the 360 would struggle to get mid-upper teens, the 318 would get low 20's on the highway with 3.55's and a 42RH that I had swapped in after about a year running around with the 998 lockup tranny....