Greeting from US.
There will be few things you may want to consider when doing upgrade with these two iMacs.
2011: The HDD is 3.5. The BIGGEST issue will be the "thermal sensor" - if you change with any other drive (no matter SSD or other HDD), the hard drive fan will RUN at full speed. Because Apple put a propriety firmware/hardware for these specific iMacs ( 2009 and 2010 iMacs share the same firmware, and 2011 iMac used different one), unless you buy the specific hard drive$$ (HDD) from Apple - you will get a Jet-engine hard drive fan to greet you when you turn the iMac on.
There are few fixes, well documented on internet:
a. Paid software ($35) called HDD Fan control for Macs (software fix) http://www.macupdate.com/app/mac/39220/hdd-fan-control
b. Free software called SSD fan control (software fix) http://exirion.net/ssdfanctrl/
c. Hardware fix: OWC made a "in-line digital thermal sensor" http://eshop.macsales.com/item/OWC/DIDIMACHDD11/
Personally, I can not speak for the software fixes since I used the hardware fix - and I have been very happy with it.
2011 iMac - very easy to remove the glass cover and the 8 screws (remember to watch out those magnets might bite your tools and screws).
However, the 4 cables attached the screen - are extremely fragile - and I saw a lot of "unsuccessful" attempts via some blogs and complaint.
I will suggest you to be very patient, in the mean time, get a 2.5 - to - 3.5 bracket to hold the SSD.
2012: It was a royal pain the properly remove the adhesive tapes between the screen and chassis.
You can refer to OWC or iFixit for how to.
I had a very good success by using this: https://eustore.ifixit.com/en/Tools/Prying-Opening/iMac-Opening-Tool.html
It is like a pizza cutter - give you a "fixed" depth in order NOT to damage the component.
There are only 2 cables between the screen and logic board ( whew!)
The 21.5 iMac has 2.5" hard drive. seamlessly for transition to SSD from my experience.
Then - another pain would be: removed the old adhesive tapes, reapply a new adhesive tapes. While lining up the screen chassis, I only have 1 shot to re-attach the cables plus re-attach the screen. You better have a second person to hold the screen to do that, matter of fact - I would highly suggest you to have two persons to do both 2011 and 2012 iMacs.
Now, back to your question: which SSD?
Because both iMacs took me a lot of time to get to hard drive - I put the most capacity to my need, in my case: I chose 500 Gb SSD.
All the SSD you mentioned are quite reputable - although you will find some negative feedback here and there.
Do some research - the firmware of these SSDs - any issues? The successful rate/happy feedback - especially on Macs/iMacs.
Can the firmware be updated after all - like a year later? Some of the SSDs from other companies - that they might not update the firmware to fix some bugs. I used Crucial SSD - so far, I have been very happy with them ( I upgraded my own SSD: 5 Mac minis, 2 iMacs and 1 MacBook Pro) with MX100 and MX200 series. I cheated by using PC to update the firmware - via SATA connections - less hassle to my experience.
After I loaded OS ( all runs El Capitan) - in "terminal" - I enabled the TRIM, I think this can be important if you write/rewrite/erase the SSD a lot.
in terminal: sudo trimforce enable
You can find this command line from internet search - finally Apple supports third part SSD's TRIM after 10.10.4
Good Luck
Sam
