I agree - if it's a short hotel and back trip, the backless would get you through. More than that I would want the back. You can pack the back in checked luggage and carry on the base. That way you are sure to have a seat at your destination in case the baggage is lost, but don't have to carry the whole thing through the airport.
There is confliciting info on backless vs. highback. At least one study showed a 70% reduction in head injuries with a highback compared to backless. They mainly felt it was from poor shoulderbelt fit on a backless, so if you do use it backless, dig out the shoulderbelt positioner that came with the seat and use it (it's a plastic loop on the end of a length of strap).
Mama to three boys ('03, '05, '07)