Portable is fine; inflatable is not. You'd be better off committing to a plug & play that stays put once installed. We had one at our first house that fit four comfortably. It was going on 25 years old when we left it behind. It ran 24/7/365, which I maintain is why it lasted so long.
We have a smaller round model now and it's 220, which has its conveniences but 110 was fine for us all those years. You just have to wait 16 or more hours for it to heat up. That's why we kept ours running all the time. It was probably costing us $30 a month by the time we moved. Your rates are likely lower. Our current tub has an economy setting that we're able to work with, so you might look for that feature.
Unless you're planning to entertain, the smaller the tub, the less it's going to cost you up front and over time. Do not sacrifice comfort though. That is the whole reason to be in there. Naked, I might add, and as a rule. Unless you like basting in laundry detergent residue.