RAM help

So I'm trying to buy well RAM for my mini itx build I did a couple weeks ago and the ram was salvaged from a dying computer and so now I'm seeing random drops in performance in my system blah blah blah 4gb hardware reserved blah blah blah

I'm looking at this ram

It's 2133 but I want to know if I should go for the single dimm or dual dimms? (Side note I chose this memory specifically because its low profile)

If possible, go dual dimms just to get that "Dual-Channel"

How about this 2 sticks of 4gb @2133 or one stick of 8gb @2400
They are the SAME price