You really can flip a coin there; Donnelly just has the sense not to say stupid things about rape, and right there you have most of the difference on abortion between the two.
But this still would be a big win. One vote fewer in the Senate to mindlessly obstruct legislation in Obama's second term. One more vote to confirm any Supreme Court appointments in the next six year. And if you want to defend Roe v. Wade, every D in the Senate is critical. I can't imagine Donnelly taking a single-issue stand against an Obama nominee just to burnish his credentials as a centrist or pro-lifer.
Mourdock is such an idiot that he actually spent state taxpayer dollars fighting the auto bailout and the preservation of factory jobs in Indiana. He's the worst kind of teahadist, and he's so dangerous that I played Republican for a day to vote for Lugar in the open primary - my fear that he could actually take office was greater than my hope that we could pick up a seat, and most of the Democrats I know did the same. So my rejoicing is great and, I think, well-founded!