I have a 2009 2.26GHz mbp with 160g HD and 8mg of ram. Am I better off just replacing the HD with a Samsung SSD 840 none pro 250G or will i get better performance with the 256g pro? The difference is about $80.00.
With a 2009 SATA II connection, you're going to be maxed out well-below the capabilities of any current SSD. Thus the only reason you'd want the Pro is because its MLC as opposed to TLC, which theoretically means that it'll be more reliable for a longer period of time. But it's not likely to make a real world difference unless to do TONS of writes on a regular basis or plan on keeping your 2009 MBP for more than another 5 years or so.
Since you're stuck on SATA II, speed differences between the 840 and 840 pro are minimal. However, the 840 pro is more reliable than the 840 because it uses MLC instead of TLC flash. MLC writes 2 bits per "cell" while TLC writes 3 bits per "cell." As a result, TLC wears out faster.