The differences between poteram and potui will be much the same as those between eram and fui, so far as temporal aspect goes. For more particular differences, here are a couple of examples:
Relinquebatur una per Sequanos via, qua Sequanis invitis propter angustias ire non poterant.
"There was left one way, [namely] through the Sequani, by which, on account of its narrowness, they could not pass without the consent of the Sequani."
Here the perfect would mean that the Sequani forbade their passage, and that was that. The imperfect implies that there was a chance of getting approval - it's not that they "were not able to pass", but "would not be able".
Nam hoc toto proelio, cum ab hora septima ad vesperum pugnatum sit, aversum hostem videre nemo potuit.
"For during the whole of this battle, although the fight lasted from the seventh hour [i.e. 12 (noon) 1 P. M.] to eventide, no one could see an enemy with his back turned."
It's a fact, it happened. Perfective. This, however, could be put in the imperfect if you wanted to talk about what was happening while no one could see one another (cum aversum hostem videre nemo poterat).
(Translations by W. A. McDevitte and W. S. Bohn)
mihi iussa capessere fas est