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I've ranted about this before (perhaps this will be the last time):
AmpHour values stated for batteries used for high current loads (such as handheld power tools) are mythical numbers.
If you define "depletion" of the battery as the point at which its terminals reach some minimum voltage (usually 10.5V for a 12VDC battery), then the amount of energy (in amp-hours) that you can get from any given battery at any given time is completely dependent on factors that are unrelated to the design or construction of the battery. Minor factors are temperature (the cooler the better), charge rate (the slower the better), and production tolerancess. The major factor, however, is load rate.
Published AH rating are generally based on a "C5" load rate, which means a load of 5% of the nominal capacity of the battery. (Sometimes known as "20 hour" rate.) Actual usage AH decays drastically as the load rate increases, and the load imposed by most hand-held power tools is not a fraction, but rather a multiple, of the battery's nominal AH rate.
We could take two perfectly identical batteries and give one to a guy driving 1-in #6 wood screws into pre-drilled holes and the other to a guy driving 1/4" lags without pilots; the usable AH of the two batteries will differ so much as to make the nominal values, as I said, utterly meaningless.
Buy a name brand tool, recharge your batteries before they completely stop, and have enough batteries so you can use a slow charger. You'll get the best performance that technology permits. Don't worry about nominal AH values.