I would go for the DX7 over the DX6i. The DX7 has 20 model memory versus 10 for the DX6i. I've looked at the features and I think the 7 has enough extra capabilities to make it worth the smalldifference in price. If you add four DS821 servos to the DX6i you are within $20 or $30 of the DX7 with the same servos.

I agree with Ken that having a lot of channels is a big deal if you really start to "scale out" a warbird.