It's easier to put a straightbore carb on a spreadbore manifold than the other way around. There are adaptors that will convert one to the other, but most spreadbore manifolds have dual bolt patterns--if you verify that the gasket will seal, you could just drop on a straightbore carb.
There has been some talk that the Eddy RPM Spreadbore has slightly higher performance potential because of a larger plenum. Personally, I don't think you could find the difference on a dyno or at the track. For me, I chose the spreadbore RPM because I wanted the option of using either style carb.