I haven't used either, but I'd guess there's not a huge difference in the quality of the microphone preamps, at least in most home recording circumstances. Maybe someone else can chime in. (I have a Saffire Pro 26, which is kind of a predecessor to the 18i8 - it's been fine for me.)
However, if you plan to use all 5 mics at the same time you can't use the 18i8 as your only interface, since it has just 4 mic preamps. You'd either have to use the MOTO, only, or you could use the Focusrite, but expand it with the MOTO's ADAT output into the Focusrite for 12 mic pres total. Might be worth a try if you've got space to set that up.
If you have both, and are already recording, what are you using now, and why are you asking?