AFAIK there are still no third party SSD's for the 2013-2014 MBA, so be sure to consider your future needs. Companies like OWC have been promising products "real soon now" for the past year and a half. When/if they finally become available, they'll likely be expensive.
I got the 512gb SSD for my 11" MBA and have no regrets because it's my primary computer. Others may be happy with 128gb, it's really a personal decision that only you can make.
Some people around here have insisted that your SSD will prematurely fail or degrade if you pack it too full. There's another long thread (definitely need popcorn there) where IIRC a couple people insisted that you should only use something like 75% of the available capacity. Personally, I never worried about this...