I'd take Walter McCarty in his prime over Glen Davis any day. Stick Walter with this squad and he'd flourish. Glen Davis would have been traded already if any GM in the league thought the same way some Celtic homers do. He's got no value. He's a bum. As is Von Wafer and whoever we took in the 2nd round.
If by flourish you mean he'd run around with boundless energy accomplishing little, yeah, he'd flourish. Baby has had impactful games against quality opponents in the playoffs. He's no all star, but he's no bum. You're on an island with that view. Were you not able to see that he was the only guy who could get an offensive rebound against the Lakers, or that he had an entire productive series against the Magic the prior year. Walter could have done that? Seriously? You call people that merely recognize his contributions homers while thinking Walter Freaking McCarty was better than him. Nice eye for talent. Not.
Hey, be nice.
Next, I have to agree that Waltah's length advantage over BBD would make him a stronger help defender, and with his vastly superior spot-up 3pt shooting range, floor spacing would be better offensively. Rebounding advantage to BBD, though I think Waltah in his prime was more fundamentally sound (boxing out, etc.), so that perhaps is arguable.
Overall, though, the comparison is valid, as BBD is nothing more than a role player in any system, and at $3 million, he's not much of a bargain value, either.
Regarding to OP, if both Rudy and Taj are available, I'd try like heck to get them both if there were any way possible. I actually think they'd play very well together.