I would always recommend a Freak system, as you can match the bore size to your paint on the day. It has the added advantage that you can change the length depending on conditions (see below) and if you change your marker you only have to replace the back.
As to length, that's more a personal issue & also dependant on the type of fields you usually play. I find I don't get any greater accuracy by going longer than 14" & can keep tighter with the shorter barrel, but on Sup'Air you can sort of lean a long barrel like a 16" or 18" into the sides of the barricade & get in even tighter that way.