In my experience, the B737-3-500 Rating will cost you less to complete. On completion of the TR you will be issued with a B737-3-900 + BBJ Type rating by the CAA. Howver, In order to operate the NG (the -6/7/8/900 and BBJ) you must complete a differences course which is typically an aditional 2 days ground school and 1 additional SIM session in the NG SIM. It can be done the other way round but obviously this is rearly done as it costs more.
NG or Classic line hours our debatable. I guess If you are targeting airlines with NG's they will probably favour NG time and vice versa. However, experience is experience and if the type is on your licence and the diff course has been completed you are qualified to fly the type and therefore in my view the more hours the better.
Regards
LOB