You could combine the 2 - get a TMG rating and find a gliding club with a Motor Glider. Hours flown on a TMG count towards SEP and, at around £50/hour, cost far less.
I would get to solo stage in a glider before going for the TMG rating. If you find a gliding club that does weekday courses you should be able to do that in a week for around £500.
Check out the
BGA web-site - it has a map that'll help you find a club.
OC619