Why the 840 evo!? I would recommend another drive! But I digress...
If you are good at space management and keep most of your data (not OS or programs.) on USB memory or Hard drives, you can definitely live with 120GB. However, it will take effort. Yes, keep your games on the hard drive too. It will take longer to load them initially, but use that time to make yourself a cup of tea or read a book or something...
If you are richer than you are efficient, buy the 256.
If you are more efficient than you are rich, buy the 120.
You can buy how many movie tickets with $60? or games? or steaks? Think about it...