Cool!
There are a few ways to organize this. Is it going to be a random tournament or do you need to announce your faction upon sign-up?
Generally, people see Fords of Isen as the most balanced map, but since I hold a grudge against playing only this map throughout one tournament and like variety, I'm usually in favor of including multiple maps.
It also depends on how many games you play vs each opponent. If you say best of 3, both players should get host 1 time and the 3rd game should be played on neutral host on Fords of Isen. In the first 2 games, host should choose map and offhost should choose spot. In this case, I would allow any map because both players get the chance to select their favorite map. If you only want to play one game (which I highly disadvise), it should be played on neutral host and you should preset the map when setting up the whole tournament. Maps like Fangorn, Westfold, Fords of Isen and Eastern Rohan are fairly symmetrical, so I suggest these maps.
Fangorn - Slightly in favor of FoD (Forces of Darkness), but fair enough to win any match-up.
Westfold - In favor of Forces of Darkness, but gives FoL (Forces of Light) a good chance of winning too.
Eastern Rohan - FoD can win on this map, but it favors FoL.
Fords of Isen - The standard 1vs1 map. Though it seems fairly balanced, Mordor seems the weakest army on this map.
Maps that shouldn't be part of the tournament:
Rohan - Too unbalanced, the host will get both middle farms before the offhost. FoL usually stomps FoD on this map.
Southern Ithilien - Completely imbalanced in terms of settlements and starting positions.
Northern Ithilien - See Southern Ithilien
West-Emnet - FoD usually doesn't stand a chance against FoL on this map. The map is fairly symmetrical, but FoD usually can't defend its starting farms.
Dunharrow - Heavily favors FoD. And I don't like this map.
Also I'd recommend you to livecast some games / the finals. feary and I already said we could do some casts together.