I don't think the Tower is the strongest of the three; it just has a better chance of winning on larger maps (I know it's debatable, but that's what we're here for).
1. There's always at least one meat shield.
2. Necromancers are more often found in the Underworld if it's available... at least on random maps... maybe it's just me?
3. You can level up Diplomacy through battles, and you don't have to spend money on upgrades... in the end, they join regardless of the upgrade level.
4. You might not have time, or you might, if you don't have problems with clearing out enemies and gathering resources (and there shouldn't be any problems with that for the Tower from the beginning of the game!).

Diplomacy is only given to Kire, and it's not always possible to earn it. But even if you level up Kire, you can forget about Archery and Offense - and in general, it will be a hero with stats like 3-2-12-14 (without combat skills). And even if you manage to build Magic to the fourth level, it won't have a significant impact. A well-developed Necromancer with Armor and Offense will have stats like 8-6-10-8, and will hit harder and resurrect troops well.
The Necromancer definitely levels up faster...
Regarding non-upgraded troops - they're not really troops anymore, but more like tech units.
Regarding having time... I think the Necromancer will find something interesting before the Mage finishes building Magic!