Will this brand of third-party RAM work in my Mid 2010 iMac?
The iMac Mid-2010 supports DDR3 10666 (from the Apple support site)

The first link is to the correct RAM for that Front Side Bus (FSB) speed
The second link is to a slightly quicker set of RAM, but runs on a lower FSB.

The differences between the two are characterised by the two notes above, the FSB speed and the clock timings.

In the first, the FSB speed is higher, but the timings for the RAM are slower.  In the second, the slower FSB allows the RAM to work at slightly faster timings.

If you want to know about timings, a search for RAS CAS delay is a good starting point.