The speed difference is negligible, I don't even think you could measure in ns. If you're building a server you want ECC memory, also because of the setup of that board and that it has only been tested with ECC memory you want the compatibility to be there.
As for the Memory to use, click the link I posted and check the "Tested Memory" link on the right, select 32GB and the buy whichever one of those you can find. This will ensure maximum performance ans stability.