Interesting choice. Robert Shaw's played a lot of villains, too, so I don't see why that should disqualify him. He is a little old, though. Despite his almost grandfatherly quality, I picture April as being in his forties or early fifties.
I think I've only seen Warner in ST6 (and Klingon Academy), but I think he could pull off a captain that's both avuncular and forceful when necessary.