I have all of the knives suggested here (2, 7, 9, 12 - not the RD7 but I do have an RTAK II...) and from the Becker stable I'll agree with the people proposing a 9 (instead of the 7). Since you already have a 15 and are looking for something to do wood processing with, I respectfully disagree with dresnor. The extra length of the 9 just gives you more room for a baton....still looking at the same size of wood, but the 9 will replace an axe in a pinch and has a lot of control for a knife of it's size. The long clip point of the 7 really doesn't gain much for wood processing vs. the 2, IMO. One knife? The 12, as BKNC concurs.The 9 weighs a scant 3/1000 oz. more than a 2, anyways....and lives up to it's moniker as a woods tool. Get a 2 or get a 9....that's my opinion. The 7, while it is still a pretty cool blade, just doesn't....trust me, for woods use coupled with your 15, my guess is you'll be happier with a 9 over the 2 or 7. If cash isn't a problem, the 12 is my choice for the "one knife" solution.....more usable than the 7, longer (and less poundage) than the 2....but that's me.
Seriously, get the 9. You can't not have a 9. That's how I got started on this whole thing in the first place. I have a LOT of Beckers, but the 9 was my first - and still a favorite. The 7? I modified the $#!+ out of mine.
So, 2 or 7? Get the 9. Or just chuck everything you're thinking about out the window and get a 4 before they're gone....
....actually, you can't go wrong with whatever you decide. There'll be more - it's almost a guarantee.