Brennan sounds lazy and sloppy to me, and
Brandon is so macho it's funny.
I love
Brendan and would have used it as a fn if I'd had enough sons, but IRL my son's best friend since they were 3 years old is a
Brendon, which I pronounce the same way but don't much like the look of. So I'd use it with the -o- spelling but as a mn only.