If you ever get into advanced sailplanes you will likely run into having four wing servos. A 7 or 8 channel transmitter, with the right mixing capabilities, should handle most sailplanes and powered planes since you mentioned that you do not want retracts or a lot of scale model functions.

Here are typical aircraft functions and the number of channels that I think are ideal to get maximum flexibility from the functions
2 ailerons
2 flaps (or spoilers)
1 rudder
1 or 2 elevator
1 throttle (if motorized)

Ailerons, flaps, and dual elevators can operate on one channel each (with the correct Y harnesses or other accessories) but you have more flexibility if your transmitter and receiver support having them on multiple channels.

First
Determine what kind of models you want to fly, what functions they need, and which transmitters support those functions.

Then
Try to go to a hobby shop that sells all of the ones that fit your criteria. Ask the salesperson to show you some of the progrmming. Then hold each of them. If you normally use a neck strap or transmitter tray, take it with you. See which one "feels" best. Choose based on feel and how the setup works. If you do the hobby shop visit, please buy from them. It would be pretty rude to use them for a demo and then order from an online vendor.