I have a Plextor M5Pro and I like it very much. It was as fast as the much ballyhooed Samsung 840Pro (which I have, also) at the time I bought it (after my much ballyhooed Samsung 840Pro died.) I also have a Lite-On M6S, which is very close to the M6Pro, and it performs very well.
The downside to Plextor is they don't have a very comprehensive toolbox, if that kind of stuff is important to you. The upside is the 5-year warranty... if anything ever goes wrong.
Personally, I would take the Plextor over the SanDisk, but that is just my familiarity with Plextor. You didn't specify which SanDisk SSD you were looking at, SanDisk offers quite a few, with appropriate differences in performance. I would make sure you are comparing apples to apples.
For that matter, since your notebook is only SATA2, you don't really need the feature set of the 'Pro' model anyway, any mainstream SSD will suffice (the caveat being the warranty, if that is important to you.)