I have wanted to know the same thing, so once I acquired one of each, I have compared my 1948 M&P and a 1950 OP, both equipped from their service careers with grip adapters, both 4" barrels.
If I were shooting for points: the Colt wins hands-down. Smoother action, best SA trigger pull on any DA revolver I've ever owned, better sights, more accurate, slightly heavier.
If I were carrying day in day out on duty: the M&P. Not as bulky, leaner and lighter, a much faster and more ergonomical cylinder latch for reloading, and as Muley said, better for fast DA.
When I compete against myself shooting the two alternating, the Colt always gets me the better score, and to be sure it's not just me, I've had a few friends do the same, with same results. But compared to the M&P, it does feel brick-like, and I'm not a fan of the thin unsupported ejector rod either.