The size, or gallons in this case, mainly determine how often the unit cycles on and off. The smaller the more cycling. If you plan on taking it with you smaller is better. I won't bother taking my 2 gallon with me, there is not enough room.
As far as new versus refurbished goes both are only as good as the company that stands behind them. I have purchased several pieces of equipment that are refurbished instead of new. As long as you get the same warranty I don't see the difference.
2008 Chevy Silverado 2500 Duramax Crewcab
Jeep Commander - sold
Flagstaff V-Lite WRB26