Ah stands for Amp-Hours. It is a measure of capacity. If you run at 2 amps for 3 hours you use 6 Ah. So the higher, the better. Other than that, it is just a matter of what you can afford and what your project can hold in terms of size and weight. Don't forget a good battery protection circuit when using lithium or you'll throw away tons of money.
Some basic equations:
Watts=Volts X Amps
WattHours = Volts X AmpHours
AmpHours = Amps X Hours