Hi there,
My best whishes for a start...
For batteries I always advise to go for the original parts !
Usually original parts give the power that is written on the package: e.g.: sometimes the branded batteries state 1500 mAh and from cheaper brands state: 1300mAh for the same equipment (Ampere per hour or milli Amps per hour). It means that you're 200mAh short also meaning that you'll work less time with the cheaper ones. If you don't need the laptop for hours without current then you could go for the cheap ones (also check warranty of it).
if the cheaper brand states the same amount of mAh, usually it's made in the same factory and there might be no problem.
Dell has different batteries for the same laptops: 6 cells or 9 cells. The 6 cells has lower mAh than the 9 cells.
With 9 cells you work approx. 50 % longer.
Cheers, Louise.