Gonnabe,
This is according to PPJN so it may not all be correct but this is how it's split in the UK at the moment:
British Airways: 737-300/400/500
Channex/Jet2: 737-300
bmibaby: 737-300/500
Easyjet: 737-300/700
Ryanair: 737-800
Astraeus: 737-300/700
European Aviation Charter: 737-200
Excel: 737-800
Thompsonfly: 737-500...orders for the 300 and one 800
Flyglobespan: 737-300
There maybe others that i've missed but as you can see, the classic is predominent in the UK at the minute.
Personally, i'm rated on both, having done a classic rating then NG differences. In my opinion you would be better doing the classic rating, getting cheaper hours then doing the differences course.
Good luck.