I'd lean towards Hill over Bonner, because he's more multi-dimensional, but I'll say this for Bonner...
What I'm most pleased with is not the 50% three-point shooting, but the confidence. I actually think it will translate to the playoffs. As much as he's been a bust in the playoffs, the sample size it's so significant that it can be said with 100% certainty that he can't play in the playoffs.
When the Spurs signed Splitter, I wanted Bonner dispatched from the playoff rotation. But the truth is, after Duncan and McDyess, he's the next most deserving big of being in it. In certain match-ups, he shouldn't be relied on for extended minutes, but he should still be able to be a useful player in limited minutes.
I'm not down on Splitter or discouraged at his play at all, but he shouldn't even be in the poll. He's barely played. The other four have made up the rotation off the bench, for the most part, since game one.
McDyess would definitely be the most important bench player if he were going to be a bench player in the playoffs. But I don't think he will be. I've thought all along that he was a starter being utilized off the bench as a means to limit his workload throughout the regular season. Presuming that indeed comes to pass, Hill is the most important bench player.