It's a bit of an odd one. Sidious would have very little chance to practice his sabre fighting without a support facility like the Jedi Temple.
But then, neither would Maul, and he was bloody lethal.
We have no guarantee that Force Lightning is superior to a sabre. We have only seen it used on a cripple and an untrained boy (Gl's words, by the way, though not in direct conncection with the lightning). However, by the time of the OT, Sidious is far too old and decrepit to be using a sabre, even if he did have one (though don't get me started on the whole 'Yoda using a sabre' thing...)
I would imagine that Force Lightning is a very lethal weapon, but not so good that he will conquer every Jedi with it single handed. It's sheer surprise factor is probably its greatest asset.
Let's not over-do it on how powerful Sidious is, though. he did lose in the end, after all.