IF your all up weight, with battery, motor, esc, Ubec or RX Battery, servos, RX and plane come out to 6 lbs. Then I would definately used the AXI 2826, but if it might come out a little more than 7lbs, then think about the 4120.

Example, I converted an SA Corsair, that I thought was going to hit the 6-7lb range. Nope, it didn't, it ended up in the 8lb range, so the 2826 was not enough. The 4120 was.
Also the 4120 can handle more current then the 2826, so I like to go the conservative route, and not stress things.

Not to get me wrong, I like the 2826. In fact I just converted a WM supersport 40 using that motor, and it flys great. I made a thread, and posted a pic and a video, somewhere in this glow to electric forum.

So, make it as light as possible, and see what your AUW will be.

I first weigh the parts individually to get an Idea before I begin. Another tip that I can give you is, go to home depot, get some gorilla glue. If you haven't used it , practice a little 1st. I mix it with a little water so it can kick faster. It will expand(foam up) so you need to hold the parts together with tape. I apply it when it expands, before it starts to set.
It sure beats epoxy for the weight, and you can easily sand it, but practice with it 1st.

That WM supersports 40 was put together with that gorilla glue, and it saved me on some of weight.

The SA Corsair was epoxied. I'm in the process of building another one using Gorilla glue instead.