X2 on the Samsung 840 Pro (not the 840) based on reviews and specs. I just received a 128GB 840 Pro earlier this week. It tested ok but I won't actually install it for a month or two (I got it on sale).
The 840 Pro has two more years warranty (five years) than the 840 (three years) and uses the proven MLC technology instead of the 840's new, unproven TLC technology which, in theory, will not be as durable as MLC (the technology is too new for results based on actual use to be in yet). The increased warranty alone justifies the higher price of the 840 Pro over the 840.
To get the most benefit from an SSD, you should put both the OS and your programs on the SSD. Even if you don't use but a third of the 128GB, the extra head room will help somewhat with performance, wear leveling, and leave room for expansion.
If you suffer with greed or need for speed, you could get two 64 GB SSDs and run them in RAID 0. That would dramatically increase the already lightning fast speeds of single SSDs. You could even get one 64GB now and add the second later once the funds roll in (you would have to do a clean install to set up the RAID) just so long as there isin't too much time between the two (drives in a RAID need to be indentical and wear on one might affect performance). Keep in mind running two in RAID doubles the chance for failure (and if you lose one, the other will have to be rebuilt).