What is the different?

Well, lightning absorb 20% will absorb 20% of a lightning attack. If you get hit with a 1000 damage lightning attack, this will cut it down to 800, which is 20% less.

+20 lightning absorb sucks really, since it will only absorb 20 lightning damage... With that same 1000 damage attack, you will hit with 980...

All in all, 20% aborb >> +20 absorb..