M500 or 600. Flip a coin, if it's a desktop or desktop-replacement.
The M500 has shown itself to generally be a pretty solid SSD. It's pretty clearly derived from, or was developed along-side, their enterprise drives, which shows in its performance (meh light usage performance, but takes heavy loads quite well), as well as features.
The 600 is newer, and less tested, but it's basically like the Neutron GTX, with Seagate's customized firmware. Seagate also had some influence in the actual chip design, apparently, so they aren't just throwing parts together. Given that I haven't read any rashes of problems, and that the Neutrons have all been quite reliable SSDs, I wouldn't worry much about it.
The M4 was good for when it was new and mainstream, but newer drives have so eclipsed it in performance and reliability, overall, that I would relegate it so some other use, having two newer good drives.