Trade Bishop.
Lehner is the goalie of the future, so to speak, with the better high-end potential. That necessitates holding onto. He is also on on par with Bishop in terms ability right now despite being several years his junior. There is very little to choose between the two - you could reasonably argue either is the better goalie.
What I also like about Lehner is that he can allow a weaker goal or two and turn it around nicely (see: last night vs. NYR... even vs. Boston the first goal wasn't exactly pretty although it took an unfortunate bounce). He might be mentally tougher than Bishop even as a rookie. Big Ben is a capable goalie... but when he has his rough nights where he is fighting the puck it always seems to snowball and you're in for 60 minutes of underwhelming netminding (see: vs. Tampa, vs. Toronto).
I feel Anderson stays our starter for the next ~5 years, however, effectively rendering the 'Bishop vs. Lehner for the future' conversations moot. But I still see no reason to opt for Bishop. Because he might be happier behind Andy as a #2? I see that as a dumb argument. You still want the better goalie to trade for a higher return down the road. If we then need to get a new backup to replace Lehner there are always a few available via trade or free agency. Maybe not as good as Lehner but I don't really care. As long as he is good enough to win us a few game in the regular season that is all that matters. When the real hockey arrives in April every team in this league lives and dies by its starter anyway.