I think you should make a list of the things that are normally sitting on your computer's harddrive that you find you access most of the time.
I was actually in the same predicament as you when I was choosing my first SSD between 128 and 256. I have been using a 256 GB for the last two years and it's never really been a problem. My use consists of about 46 GB of music, 3 GB of photos, and about 8 GB of apps (which includes Diablo 3). Of course theres other space being taken up such as the OS I believe is around 15-20 GB.
If money was tight, I'd be okay with a 128 GB because external storage is so much cheaper and not everything is necessarily needed to be on me 24/7. I'm actually not sure how people fill up these drives so fast, maybe it's torrenting movies or something. Just my own opinion and use case scenario, hope it helps.