Well, honestly, it just depends on you. I started this awesome hobby with the Spektrum DX6i 6 Channel transmitter and I have flown many planes and a couple of helicopters with that transmitter. I just recently upgraded to the Spektrum DX8 8 Channel transmitter because I like the feel of the rubber grips on the sides and I like that the DX8 has several 3 position switched instead of the two position switches on the DX6i. There are many other features of the DX8 that I haven't even used yet.
I can understand how you would wonder if 6 channels is enough and quite frankly, for a beginner, the 6 channels is plenty. However, if you do think that you might want to have a plane, someday, with a bomb drop (or multiple bomb droppers) then maybe you should consider getting more than 6 channel. It would be something that you can grow with and not be afraid of being limited.
The DX6i is an awesome transmitter. The DX8 is even better. This, of course, is my opinion. What should you buy? In the end....it's entirely up to you.