510 vs 520:
510 used the marvel controller. 520 uses the Sandforce SF22xx controller.
The 520 has a very good 1/2 egg vs 4/5 egg reviews @ new egg. But then so did the 510. http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Do not confuse this with true reliability as the drive has not been out long enough, but based on Intel's track record should be the best followed by Samsung.
Most High end SATA III SSDs will perform very simular to each other in REAL LIFE day-to-day usage. The diff in Sequencial benchmarks using ATTO is meaningless. Caustion on some of the LOW end sata III drives - Use the Agility III as an example (and yes I have two of them). They perform No better when on a sata II port as compared to when they are on a SATA III port.
I have 2 128 Gig M4's, one samsung 128 gig 830 and 2 120 gig Agility 3s (plus 5 of the olded generation sata II drives including Intel G1 and G2).
For the SATA III drives my choices are Intel -> Samsung 830 -> M4s. But, Of these 3, I Buy the one that is on Salle at the time of Purchase.