You might encounter two different issues here.
The first is some MacBook Pro's have a SATA II (3.0 Gb/s) optical drive port. Here's a good reference: OWC Data Doubler it makes no difference who's carrier you have as the issue is within the Mac's logic board. If you have one of the odd systems you'll need to find a older fixed speed SATA II (3.0 Gb/s) drive for the optical bay.
The second issue you can face is the Apple HD does not have crash guard onboard so moving it over to the optical bay does put it at risk if you bounce the system while the drive is running. Only the HD bay has crash guard protection. So I would leave the HD in its current position and put the SSD in the optical drive bay.
Frankly, if you have one of the odd systems you might as well just forget the dual drive config and just get a Seagate FireCuda SSHD replacing your current HD.
Here's some info on SSHD's: SSDs vs. hard drives vs. hybrids: Which storage tech is right for you?
Here's info on the Seagate FireCuda 2.5" SSHD. Just keep in mind this is a fixed SATA III (6.0 Gb/s) drive.
As for which SSD you had listed:
- I've used the Samsung 850 EVO in quite a few systems and I do find they are tops in both quality & performance!
- The Crucial SSD is also quite good from a performance perspective. I do find their documentation & support is a bit thin.
Frankly I don't have an issue with either. So its more likely an issue of cost and availability.
One point I wouldn't leave the optical drive bay empty. At least get the optical drive carrier so the slot is covered and the stiffness of the system can be maintained. This is important if you grasp the system with one hand where the optical drive would be.