What speed is the SATA controller in a July 2012 macbook pro?
Solution 1:
OWC has tested it and confirms that..
...the MacBook Pro 15” (2012) has two drive bays, each capable of running an OWC 6G SSD at full 6Gb/s speeds.
which is the SATA Revision 3.0 speed.
They've also setup a striped RAID which benchmarked at "over 1000MB/s read speeds and write speeds that nearly hit 900MB/s".
Regarding the Macbook Pro 13" (2012), OWC recommends using a 6Gb/s SSD only in the main bay, as the 6Gb/s performance of the optical bay is very poor. 3Gb/s models however work without problems in the optical bay.
Solution 2:
They've had SATA 3 (6Gb/s) since switching to Sandy Bridge early 2011. I'd say get the SATA 3 SSD if it doesn't cost much more.
If the combination of memory chips and controller inside the SSD itself can't supply data as fast as 3Gb/s, you won't ever max a SATA 2 connection. However, some current SSDs are starting to approach the real world limits of SATA 2, so they should all be on SATA 3 soon regardless.
Depending on how disk I/O intensive your work is, you may not notice the difference between an SSD on SATA 2 vs. SATA 3, so I'd say if it's a huge price difference, it won't be worth it for most users.
But you'll definitely notice the difference between a spinning disk and an SSD on any version of SATA.