I am pretty sure that radio takes a 7.2v battery. And the voltage doesnt make a battery better than another battery, a device technically only uses a certain voltage battery. Now "mah" or "mili-amp hours" is what you want to look for. A 7.2v 1400mah battery will not last as long on a single charge as a 7.2v 2000mah battery.
There is no real difference when it comes to voltage. Going from 7.2 to 7.5 means nothing. What you are looking for in a longer battery life is the current rating. If a standard OEM battery for a given radio is 1500 milliampere/Hour and there are batteries available (all over the web) then go with one which is of higher current rating.