I think this is very important to discuss.
Because if these two characters were in a battle, it would be very similar to L vs Kira, both are intelligent, and L would stop at nothing to discover a case.
A is almost impossible to find out, of whom that person is, that I had to spoil the story, by looking up the wiki to know who A is.
Now if A is able to defeat L, then he would have a backup plan
Near-whom is considered by some others to be smarter than L
Mello-Whom is a boss.
So whom do you think will win.
Remember it took like 5 or 4 books to find out who A is.