More cells = longer life = more cost so if you want longer battery hours you will need the 12 cell rather than the 9 cell.
Don't pay for brand name eg Dell Toshiba etc. as batteries are often just re-branded and price increased.
I bought a cheap £16 for my Toshiba last year and its just as good as the original.
Make sure you go through the correct charge re-charge cycles to get the best from your battery if you do not do the first cycle correctly then then battery will not last long at all.