It has a lot of smaller mods and it used to have mystcraft, which I want back to be officially included again ;-;
There's isn't really a big difference right now, but I would expect the developers of Voltz to be trying to make and find new things to add so it's not a 99% copy of the universal electricity pack.
Mystcraft is amazing, btw. And I'm really kind of assuming that there's going to be more mods added in the future...