I thought this thread had finished ages ago. My personal summary goes like this.
As a day fighter then it has to be the P38.
As a nightfighter then its the Mossie. I know the USA had NF P38's but no one can claim it was a 'standard P38', it was also more than a little tight in the back nor was it as effective or widely used. The mossie was also a special built but it could be argued that it required less modification than the P38. It already had a crew of two and internal space for the electrics. However at the end of the day, they were both modifications.
As a GA its so close as to be a draw. Mossie had considerably more firepower, and the ability to carry rockets as well as 1000lb bombs internally whereas the P38 had extra performance, sufficient firepower and similar payload.
As a pure bomber the Mossie has it on range, payload and speed, plus the not insignificant fact, that it carried a bomb aimer and is far more likely to hit the target. The P38 could carry a bomb aimer but then it loses its guns and is also a special build. A standard P38 could carry the same payload certainly but this would all be hanging off the wings and have a significant impact on performance, both speed and range. It also lacked the ability to carry a 4000lb cookie and be far more likely to miss which defeats that object.
Thats my summary for what its worth.