128gb should suffice for you, if you consider yourself a fairly light user.
I am using the 64gb macbook air and am down to around 10+gb of storage. What Apple doesn't tell you is that in addition to the OS, it will also use up about 5gb of storage for the mail app, 5gb for photo stream, and 4-8gb for sleep disk images, amongst other misc odds and ends.
Apps like Office also take up space. If you use dropbox, that can also take up space if you choose to keep everything. There were a few times when I came dangerously close to running out of storage (left with 1-2gb of storage), like when I was upgrading to Mavericks and some server bug led to me downloading the update (~4gb) twice. I have no music or movies on my macbook.
If you want to edit a video clip using iMovie, that's pretty much out of the question since a 45-min clip filmed on my iPhone already takes up 9gb of space. That and I find external storage quite cumbersome, what more since it is so easy to just close the lid of the macbook and walk away when it's time to pack up and go.
In hindsight, I should have gone with the 128gb one, but what's done is done.
