2 Avern
The thing is, in this case, comparing units one-on-one isn't entirely accurate. Because Necromancer's skeletons destroy a lot of things. For example, they take griffon conservatories and hives very early on, where they supplement their army with angels and wyverns. And they can also take Utopia very early, which provides gold and artifacts... So, by the time they meet, the Necromancer's army and stats are usually noticeably stronger than Fortress'... Of course, Fortress has a library, and their magic is usually good, plus early on, gremlins provide a lot of offensive power, but overall, Necromancer is still somewhat preferable (due to the number of skeletons, which translates into better hero stats).
That's why there aren't too many people who want to play Fortress against Necropolis...