Such a mode could result in purchases, granted. I’m saying that it will be substantially less because the player-conflict element is gone. Until the last century we used to go to war as ostentatiously as possible in order to intimidate the enemy. That’s the drive that makes you want to stand out.
Then why would you even want to present players with the choice between a mode that will make them purchase more, and a mode that will make them purchase less? People are sheep, they have no idea what they want, if you present them with one option they will even feel better than when you don’t impose the responsibility to choice on them.
Even if both sides were equally profitable, then it would still be better to limit it to one mode or at least very similar modes, that way people have the idea they’re playing the same game and you don’t end up in a fractured community. In that TF2’s horde mode was a very risky move and we’ve yet to see the full consequences to the playerbase of that.
That’s why I say, leave the options to spent the time in a less consumptive way out and only give them the way in which they will spent the most money.