Personally I'd keep the ultrabook and build a desktop for gaming instead of having to lug a heavy gaming laptop around. But that also means you can't play games on-the-go, if that matters to you.
Any i5 or i7 machine is fine for gaming. Minimum 8GB of RAM. You can buy used parts and put the money you save towards the best video card you can afford. Everything newer than Sandy Bridge feels pretty much the same speed. Get a Samsung SSD too.
I can feel the difference between sandybridge and haswell..
It's probably due to the increased memory bandwidth making everything just a tad more snappy.
This is assuming both are clocked equivalently and LOCKED..
If the clocks are not locked or minimum cpu ratio set, the sandy will actually feel snappier than the haswell, because sandy minimum is 1600mhz while haswell goes to 800..
@ 800, haswell will choke on certain things that suddenly puts a demand on memory/ cpu performance.
Overall, you wouldn't buy a sandy TODAY, because it'd pretty much cost you the same as a new Haswell..
You might as well get haswell..
UNLESS you're a n00ber and is too afraid to delid, because haswell can't OC very reliably without delidding..
Only very expert people will be able to OC a Haswell without delidding, it's a serious PIA to cool it.