I would like them to be a PvP-PvE mixture. Players can either do PvE in an area guarded by NPCs to farm ressources, build the base, clear NPC enemies, etc. Those ressources then get used by the PvP players, which are granted buffs and additional NPC corps the more the PvE crew builds their base. Those players then advance on the enemy, which consists of players and their NPC troops as well.
In essence if all players choose to got PvE, then the NPC troops get the strongest and the warfront will be an economical battle. If all players go PvP, then it will be like a regular epic battleground.
There could be a mixture, where players could do PvE around captured outposts in the PvP area to fortify them. Those players are not guarded though and thus susceptible to enemy attacks.
I would make it 40vs40 or (for a nicer number) 50vs50.