Dishwashers are a luxury, but they also use a LOT less water than you'll use manually washing dishes and (as long as you clean their filters and use the right detergent) generally do about as well as a bored person doing the dishes will do (ie every now and then you'll find one with gunk but not too frequently).
Personally, my household has 3 adults and I would go for the full size dishwasher and figure something else out for storage ... It's also possible the smaller ones might be perfect for baking sheets and whatnot. If you haven't, try to do a mock layout with painters/masking tape and see what you can make fit in that space to try and help with planning, if you can.
With all that said, worst case scenario if no one wants to manually wash dishes, you have to run it twice in one day, every now and then, which doesn't seem horrible.