[peter tours]"Please suggest the best for the purpose. It has to be SATA mount (drive bay) as my slots are full or would go w PCIE mount."
PCIe would give you several benefits. I bought an OWC Mercury Accelsior E2 PCI Express SSD and not only is the SSD incredibly fast, it also adds 6Gb SATA III to my 2010 Mac Pro via eSATA ports. So my external RAID 5 is connected via 6GB eSATA. I realize that you don't have an empty PCIe slot but I just wanted to give you something to weight your options against. Maybe one of the PCIe cards is not as beneficial as getting 6GB SATA III.
If you want to stay with your internal SATA II bays, then the Samsung EVO SSD's are good performers. I have the Samsung 850 EVO Pro in my 2008 Mac Pro and it's working out great. Even though it runs at 3GB/s, it was like getting a new Mac when I put it in compared to an HDD. It really made a huge difference.
[peter tours]"Is there any benefit to buying a 6gbs SSD when my system is only 3gbs?"
Nope, not if you are using the internal SATA II bays. OWC probably has some older 3GB/s SSD's for sale but most everyone else is only selling the 6GB/s SSD's these days.
[peter tours]"When I install the SSD I will also be updating from Mavericks to El Capitan which I understand puts the TRIM issue to rest."
I'm not sure that will happen automatically. You may still need to use the trimforce command to manually enable it. Here is an article on how to accomplish this: