If it's for a robot i'd say go with the voltage nearest to your motor specs. Regulate it down for your circuitry.
If it's for circuitry alone I'd still regulate it down but pick a battery closer to the total voltage required by your regulator i.e. 7.2or6.0 for NimH, 7.4 for lipo.
As for capacity, work out the total draw for your robot/cicuit and multiply it by the desired run time, Then look at capaities you can actually afford.