which ipod mini battery to get?

i have been checking to get a new battery as the one is not staying chaged and mini is only 18 months old. i have found batteries they say is for ipod mini but they vary. they are all li ion 3.7 volt but the mah varies i have seen a 500mah which they say will run 4-6 which is comparible to original then there is a 600mah which runs 6-8 then staples has one and it is a 720mah. any idea which one to get macmall has one from sonnet which is a 600mah.

thanks