I'm in FL and have a similar size GD 5er. We're adding the 2nd A/C before the summer simply because it's difficult for the one unit to keep up on those 90+ degree days. But we always camp with electricity and do most of our camping in the south.
So in Wisconsin, I can't profess to know the climate, but if frequently camping without electricity hookup and staying mostly in the north...I honestly can't see the reason to have the 2nd A/C. I would prefer the vent.
You say it's the same price, but how can that be in reality? Are the units 100% identical? For our GD, there are different layouts and features within our same model number. Is the dealer having trouble moving the rig with the 2nd A/C? Perhaps you could negotiate a lower price on the single A/C unit. Also, what type of unit is installed in the bedroom? If it's not a low-profile unit, it'll look like a giant tumor sitting up there and increase your overall height quite a bit. I saw one recently on the GD FB page where the new owners had a 2nd A/C added up front and it was a full-size exterior unit...talk about UGLY!