2011 iMac 2.7GHz i5 - Hard Drive Upgrade?

There's an extra SATA connector on your iMac's logic board. You could just add the SSD and keep the existing HDD. I did just that using OWCs guide that you can see here. Very happy with the results.

You can also use this guide to simply replace the HDD with a SSD. You might want to also read up on fan control if you replace the original HDD.

 

Samsung 840pro, the best

 

The Samsung is likely the best. I used the OWC Mercury EXTREME Pro 6G when I did mine. I will say that it works really fine... except that OWC came out with a firmware upgrade for this SSD, and their upgrade program and method for my 2011 imac doesn't work. Still waiting for a resolution from them on this issue. (OWC states the firmware is recommended, but not required. But, I like having the latest firmware on such things)