I would recommend you hold off for now. The new MBPs are just around the corner. Personally, I wouldn't advise you to upgrade your internal Air SSD. It's almost a waste since you can't use your older SSD for anything (nobody wants a 128GB external drive). Besides, the prices are way too high as is. However, if your storage needs are urgent and can't be fulfilled with an external HDD, go for an rMBP (though try to push it till the end of the month).
Regards
Raptor