The effective levels of undead that can be turned with these spells are:
Spell
Default
Necromage
Turn Lesser Undead
6
7
Turn Undead
13
16
Turn Greater Undead
21
26
The durations are the same for all spell levels, varying only with perks applying (remember, for dual casting, you need both the perk and to be dual casting it):
The "Turn Undead Push" effect varies between these three spells. Notably in the "Turn Undead" spell, it has a magnitude of 0. This means it may stagger an opponent, but won't push them back.
The Unofficial Skyrim Patch, version 1.3.0, addresses this issue. The stagger effect for Turn Undead has been set to 75 pts.