In practical terms I would say not a great deal of difference in performance but some better battery life.
The 750M is most likely and probably only slightly faster. 30% max.
The Haswell will not be significantly faster either. A little more IPC, slightly higher clocks otherwise Intel most focuses on power consumption at idle.
I wonder if IGZO displays will be ready by then. That should greatly improve battery life.
The 13" should be more interesting. If one looks at the temps reported in reviews, a 35W Quad Core should have been an option. Anand says it is too expensive but that is a 2000$ notebook
.
I think the only reason they didn't put a Quad in is because they wanted to hold something back that sounds like a great improvement the next refresh. When they can claim Quad Core + double GPU performance. Otherwise the cooling system looks like it is more than up to the task of handling a 3612QM and the battery life would survive it too.
The 13" this time around is a poor buy IMO. If one doesn't need speed than a slimmer lighter ultrabook with an ULV CPU would be better anyway.