Is this nitro or E-Buggy? If nitro I would go for the HV so you can run a 2S lipo rx pack and not have to worry about cooking the servo. If its E-Buggy then you can probably set the voltage from the ESC so thats not such a drama. If its E-Buggy then choose whatever suits your budget, specs etc.

I have an MT4S and find the tx to be a decent size, not particularly small like a Futaba 3PV for example. I've seen the MT44 and MTS and they don't look that small to me, they look similar to my MT4S. Can you check out someones from the track to see how it feels in your hand?