Like Mark says, see if anyone nearby can rent out a 12" for a day. Otherwise, go with the 6".
If you're half decent with a hand plane you can take the seam off with one for the glossy look.
In my experience, portable planers are great for sizing, but not the best for putting the finishing touch to a timber, unless you're planing on sanding it.
And the 6" Makita is well worth the investment for any future projects...
Good Luck!
*K
