I can't say i'd feel overly confident with either of them that high in a PPR. I'd probably consider Ronnie Brown a better PPR option, and probably even Rudi and FWP. With that said, if i had to choose between the two i'd still go Alexander.
I'm not really down on Portis next year, but I do think that he's going to see significantly less touches in 07 as he has in years past. Betts is the better receiver and better in protection, so he's going to be on the field a fair amount considering how good he's looked to end the season. Do you want to take a guy with the 7th pick who isn't guaranteed to see a majority of his teams touches? It's not like the Skins are the Chargers here either. They've got Campbell leading an offense that figures to be fairly conservative next season, which'll obviously limit his scoring as well.
Alexander on the other hand will again have the job to himself. He's not a great receiving threat, but will still catch his 15 to 20 balls. The Seahawks offense will again be upper tier, so considering Alexander's nose for the endzone, he'll more than make up for his lack of receptions by hitting paydirt. I'm concerned with the explosiveness i've seen, or lack thereof of him to this point though. Even before the injury, it sure looked like he carried the ball 370 times in 05, as he just didn't have a lot in the tank. Sure he went off for 200, but with 40 plus carries. This isn't the SA we saw in 05, so even in a non PPR, I don't see a top 3 back here.