Just as in cars, hp. figures don't mean much taken by themselves. The Supertigre makes that power at an rpm the Saito never sees, and the Saito probably out torques the ST. As far as superiority goes, a matter of opinion, but they don't get any better than Saito. You can turn a larger diameter prop with the 4-stroke engine (Saito) and use less fuel doing it.

I don't know which P-51 you have, but either engine will work on a .60 size plane. Some just like 2-strokes better than 4-strokes, but (and I am prejudiced here) nothing sounds better than a 4-stroke in a warbird (or any other plane, as far as that goes) I own nothing but Saitos, not counting my gasoline engines.

And generally, 4-strokes are more expensive, more moving parts and more complex.