I'd probably prefer the Islanders, though there's pro's and cons to both.
I think the Bruins and Islanders play a similar brand of hockey. Defensive, low event hockey. The Bruins have the higher end talent so theoretically that should give them the advantage. But playing a bunch of tight games can lend itself to more randomness in a series. A bunch of 2-1 or 3-2 nail biters where the bounce of a puck is the difference is a scary proposition. We also struggled with Varlamov this year, though Varlamov got hurt and then sucked and right now it's Sorokin's net (and I assume if they get past Pittsburgh they'll stick with Sorokin).
Pittsburgh has the stars and fire power but are weaker in net and defensively. Those games could turn into track meets, and the Bruins have the fire power to match (if not have more). The Bruins have also matched up well against Pittsburgh in the Crosby/Malkin era. On the con side, Pittsburgh would have home ice.
Tough call. Honestly, I don't really care. I think the Bruins at their best are better than both Pittsburgh or the Islanders at their respective bests.