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It all depends from what type of application you are gone to use them.
on RC models they use NI-CD which have a fast discharge rate, i mean than they let out all the current they have in a short range off time, so they are good if you have to boost much current from them.
The Ni-CD don't have a long durabilty of the charge (all at once ) and you MUST discharge them completely before recharging them other wise they wont last long.
Ni-Mh instead have a more steadyo utput of the current, have a low dispersion so you don't nead to charge them so often and you may charge them almost at any point of the battery level.