Im in the same pickle. I bought my first macbook last week. I opted for the 128gb Macbook Pro Retina version and was on the fence if I needed the 256gb instead.
I am strictly using it for:
Basic web browsing.
Microsoft Excel/Word for senior year of college
Almost no gaming
I don't use iTunes (Spotify) and I DO NOT have many photos.
I do want this machine to last me 2+ years without any regrets...So I am worried the 128gb will last me long enough. Even though you can simply purchase an external chip to store anything else. Decisions.....decisions..
If you are looking for just 2-3 years in future, I would suggest you to stick to the 128GB and buy an external drive. External HD or SSD with USB 3.0 and beyond are excellent. I hardly get to notice any big lag in transfer speeds etc while using it. Just store the unwanted movies, songs etc in that and you will be good to go with 128GB itself.
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Forgot to mention in the earlier post... I too bought a 128GB around 12 days back with a 1 TB external Hard drive.. I too was in a dilemma... But later thought that for the difference I would pay for the 256GB, I might as well go and buy good external drives with multitudes of space than the mere 128GB extra that I would be getting for $200 more...
And if you are planning to use the machine for 3 years and above, even 256 GB will be small if you are big on storing media files (photos, songs, movies etc)