I have never heard of neutering changing a dogs personality. When I had my boy neutered, he did gain weight, however, his energy level did not change! He is get go from the morning until night. Wants to play, play, play. And, if your boy is marking now, it is quite likely that he will mark after being neutered, just prepare yourself for that. My boy still marks
I agree, it is very hard if you have an unspayed female in the house, that is all the boy can think about. I do not regret having my male spayed, I do regret I didn't do it sooner, before he started marking.
Good luck.