It seems like a stretch to me that both would be in the NHL. I'd say Letang has the better chance, based on last year. But also based on physique.
I never quite seem to see Russell when he's playing at his god-like dominating best, so maybe I'm missing something in my analysis on him. But from the games I have seen, I still have a lot of trouble imagining that he could survive in the NHL until he can add some additional strength and bulk. Columbus may not have the most talented team in the league, but still, as far as I know anyway, they're going to have Klesla, Foote, Hainsey, and Westcott back. Plus Tollefsen and Johnson, you'd think. My impression was that Eriksson played well for them, and I'd have thought he'd be back in their plans, but I guess I don't know it. Plus now they've signed Maenpaa? There's a lot of bodies kicking around on D, anyway. Maenpaa and Westcott are pretty smallish too. Rushing Russell in doesn't *seem* like a good bet to me, anyway.
Whereas Letang I think is already thicker and stronger than Russell, and I can see him surviving better in the NHL, plus there seem to be more openings on the Pittsburgh D for a young kid to come in and play a bit. Gonchar, Whitney, Orpik and Eaton seem to be the only sure bets there. The Pens will probably try to sign a somewhat-prominent free agent on D. But there will still be a couple spots available after that.
So Letang wins on the "better right now" bit... at least in terms of being more ready and able to step immediately into the NHL.
Longer term... no idea really. Russell seems to be recognized as the more purely talented guy, so presumably as long as he makes the requisite physical adjustments, he'll ultimately end up putting more points on the board. In terms of style, I tend to see Russell as the guy who's better at skating with the puck, joining in the offense on the rush, or pinching in on the point. Letang may have a shooting edge, but I don't expect him to be a notorious puck-rushing guy like Russell could be. Defensively, if Russell truly can utilize his speed the way Niedermayer does, he could be very effective there too... when he gets burned, it tends to be getting caught on offense or being out-muscled, but overall he plays his position better than Letang IMHO, and he has a fair bit of feistiness that might put him in good standing if he truly can add the strength to back it up. So longterm, I guess I'll go with Russell.