That SDXC slot speed is slow in comparison to the internal SSD. It's really intended for data only, your collection of pictures etc. not for anything OS related, Apps etc. But you got the 11" moot point.
Now a newbie doesn't know how much room he needs but you came from a PC, you should know how much room you require. Planning to do something new?
I lived with a 80 GB Sony laptop for 10 years and it was 60% full for the whole time, so I KNEW a 128 GB Air is what the doctor ordered. All my Big Stuff are on my home server and I have no need to lug them around with me on my portable. As someone coming from Windows too, I entertained loading Windows on the Air (256 GB here would be better) but as times goes by, and I slowly porting Windows Apps to OSX, I feel less and less need to run Windows on the Air. It's been one year and my 128 GB is at 36% full.