850 is good for the price. The only thing better about the PRO is that it has 60 years of storage life and the 850 only has 19 years...but I mean really, are you still going to be using this in 19 years? Unless you're building a super high-end machine, that SSD is more than enough. Most SSD's labelled SATA are SATA III, some of them are SATA II. Look for it to have a speed of 6gb/s that will tell you that it's SATA III. 250gb and 256 gb models are exactly the same, most drives are a little smaller than advertised, the 250 is probably at 256 but they advertise it as a 250 because they know they are always a couple gigs short. Hope this helps.
Enjoy!