I agree, if you want to be a pilot then it wont make much difference, so go with the option you enjoy most that way you will get a higher grade and find the course easier and your time at college more enjoyable. If you go onto degree level the course you do at uni will probably include both avionics and mechanics anyway. I presume you are doing physics AS level so thick about which subjects you find easiest i.e mechanics is force = mass x acceleration, speed, workdone etc. Avionics will be more about current, voltage, frequency, resistance, wavelength etc. You should know which type you find most interesting. Have a chat with your physics teacher and more importantly be mature and give the college tutors a ring to discuss the course in more detail. Also think about your back up plan, what do you want to do if your not a pilot. If its aircraft design or maintenance including powerplants (engines) then mechanical is better, if its design or maintenance of systems (navigation, electrical, communications even inflight entertainment) then avionics will be better.
Good Luck
HL