Williams is obviously going to be better than Bruce going forward but Bruce is better now, and, in theory, can more easily handle being a bench player (though there's no actual evidence of that other than him being a veteran).
Hard to know what to think of it until we see how the pieces actually fit together, but the bottom line is, the Phillies already preferred having Gosselin and Rodriguez to having Willliams, so Bruce is an upgrade there. It's the Matt Stairs role, quite simply.
And Williams isn't really, and probably won't ever be, a pure power bat. And if he's not in the trade sending him back to AAA is still a possibility.
Bruce didn't have a hot streak early in the year though; his OPS in May is higher than it was in March/April (when he also hit .187). Fewer HRs in May - otherwise he'd be leading the AL - but more SLG overall.