Bishop is still better. That's a pretty good reason.
If we can hang onto both, I'm all for it. But if we have to move one, I'd rather not move Bishop and then wait 4-6 years for Vasy to hit his prime, losing games in the meantime.
This was my position right up until Bishop got injured yet again, followed by the announcement that the cap won't be going up. With a cap increase of ~$3M per this offseason and next we probably could have kept our entire core together; now if we keep Bishop we're probably looking at losing a couple of our key forwards. Considering the problems we've with scoring depth already this would effectively turn us into the Rangers: great goalie with nobody in front of him. Not to mention Bish's injury issues and the massive threat that poses going forward. If he declines then not only will we have no offense, we'll have no goalie either, and no cap space to rectify either situation.
Moving forward with Vasy is a risk, but it's a risk that at least gives us the flexibility to fix things if they go south on us. With such a tight cap we simply can't afford to have players like Stamkos and Bishop on massive longterm contracts. Hedman and Kucherov, sure, but not two players with injury issues, one of whom would be highly unlikely to live up to his contract even if he can stay healthy. Plus moving guys like Bishop (and if we want to get a jump start on the youth movement, Garrison) would also allow us to replenish the coffers in Syracuse with some high draft picks and/or quality prospects (or we could keep those guys for one last run at the Cup, which might be the more popular option even if we'd benefit more in the long run by moving them.)
Basically the cap situation changes things completely. Before our window could have remained open uninterrupted; now we either go younger and cheaper to retain as much of our core as possible and have a longer window open down the road, or we hold onto everybody as long as we can and watch the team break apart over the next few years.