Samsung's 840 Pro and OCZ's Vector really push the SATA 6Gb/s interface to its limits. Frankly, it's hard to imagine getting much more performance from today's storage interfaces. Enthusiasts who need more speed in the desktop space can explore RAID, go with a PCI Express-based SSD (though that market remains a niche with a big premium on it), or wait for SATA Express. http://www.tomshardware.com/reviews/ssd-recommendation-...
The main difference is Write speeds and the flash being used.
Write speeds are self explanatory, the Pro gets around 450MB/s while the Vanilla 840 gets around 160MB/s. Read speeds are about the same at 450MB/s.
The flash used in the Pro is MLC, while whats used in the Vanilla 840 is cheaper TLC. The MLC is what lets the Pro have a better write speed, and also stands up to far more rewrite cycles before it begins to degrade. However I wouldn't be concerned about that, any modern SSD will last well past its obsolescence before it starts to degrade from normal usage.
So basically, do you need better write performance? If your just using it as a boot/programs drive, then IMO you dont.
840 Pro is really high performance, usually also the 840 will do a good job, only write performance on the 120 GB drive is a little low, but this is not so important for everydays use. If you wanna save money go for 840 and enjoy SSD speed.
My budget meant I had to go with a 250gb 840 rather than a pro. The read speeds are virtually identical on paper. I sometimes regret not waiting and getting the pro but that is only when I'm sat using programs like Winrar to open, decompress and then write gamedata's that can sometimes be over 4 gb's in size. That takes time and I sit there drumming my fingers wondering just how noticably quicker the pro would be. It may not be, I haven't owned a pro to say for sure. Anyway let's get back to my 250 gb 840. Boot up times are great, Infact everything is except what I just mentioned.
The sequential write speeds for a regular 840 are:
120 gb up to 130
250 gb up to 250
500 gb up to 330
The sequential reads are 530 on the 120 gb and 540 on the other two.
For the 840 pro the reads are the same and the writes are:
120 gb up to 390
250 gb up to 520
500 gb up to 520
There all of the box stats. I tested my 250 gb 840 and got results that were close enough to let me say the differences were negligible.