Leslie really offered a fine analysis here and I'll take it a bit further.

Do you want to square timbers, smooth timbers, or straighten timbers?

If you want them square I'd have them planed at the mill.

If you just want to smooth I'd buy the 12". With sharp (disposable) blades it virtually glides down the timber and is easy on the body. The roller on the front helps tremendously with positioning and saves your elbow. It's an amazing machine.

If you want to straighten I'd go with the 6". Once straight you can smooth any ridges with a hand plane or smaller power plane. I like that these blades are not disposable.