The bishops for sure. You say the knights can protect each other which is of course true. But when they do this they end up rather frozen in position and ineffective because they are tied to the function of defending each other. The bishops influence more of the board and are able to control from a distance.
2B vs 2N or 1B vs 1N
Sort:
In an endgame if you had 2 bishops and your opponent had 2 knights do you think it is better to trade one bishop for 1 knight or keep the pairs?
Obviously the answer would depend on a heap of other variables but all other things being equal.
The knights can protect each other which the bishops cannot but the bishops are more mobile on an open board.
Trading down to 1 vs 1 retains remaining bishop mobility but removes the knights ability to protect one another.