raisonjohn :
To save money and for purely a system drive and some programs (no games, other applications whatsoever), the 120GB is more than enough.
A SSD is basically a bunch of flash dies operating together as a big RAID 0. The 120GB SSDs typically have half the number of flash dies as the 240GB SSDs. Read operations are fast enough that they're not as affected as much, but write operations are significantly impacted. A 120GB SSD's 4k and 512k random write speeds are almost half that of a 240 GB SSD.
http://www.anandtech.com/show/8747/samsung-ssd-850-evo-review/8
For this reason, I highly recommend paying the extra $30-$40 for a 240 GB SSD. The extra space is just gravy.
It may seem like a lot to pay for "only" an extra 100-150 MB/s. But you have to understand that your perception of a SSD's speed is the
inverse of MB/s. MB/s is how much data can the drive transfer in 1 sec. But you perceive speed in terms of how much time it takes for the drive to transfer x MB. Consequently, speed improvements at the lower MB/s ranges matter a lot more than at the higher MB/s.
e.g. Say you need to write 1 GB.
170 MB/s (120GB EVO 850) = 5.9 sec
320 MB/s (250GB EVO 850) = 3.1 sec, a 2.8 sec improvement
470 MB/s (+150 MB/s) = 2.1 sec, a 1 sec improvement, 36% of the first bump
620 MB/s (+150 MB/s) = 1.6 sec, a 0.5 sec improvement, 18% of the first bump
770 MB/s (+150 MB/s) = 1.3 sec, a 0.3 sec improvement, 11% of the first bump
As MB/s gets higher, each +150 MB/s results in a smaller time savings. So it's the first +150 MB/s, the one you get moving from a 120GB SSD to a 250GB SSD, that is the most important.