I own a first generation Focusrite Scarlett 18i20.
I can't really understand the need of very low latency when recording,
because the Scarlett has got an internal mixer, which can monitor the
input signal nearly at 0 latency. And I think that the Presonus
doesn't have an internal mixer, or at least it's not controllable from
a linux machine (the first generation Scarlett mixer is controllable
since kernel 3.19 thanks to Robin Gareus' reverse ingeneering work of
the protocol).
Of course if you use virtual instruments latency is important, but I
always used the Scarlett as a simple recorder, so I cannot tell if it
works, I usually use a PCIe RME card when using virtual instruments
and it works well even with vanilla kernels and threadirqs, I get some
xruns when I use some heavy cpu virtual instrument sometimes.
Ciao
Guido