Butts or Ribs on top?

well, it's just preference. Do you use the same rub on ribs and butts, that's probably your biggest question. If it's the same, it shouldn't matter.

You'll have racks of ribs dripping on the butts or vice versa.

Do you wrap your butts, then it's a non issue. You could wrap then then it won't matter.

Only question might be that the excessive dripping, will it matter (good or bad)

Me, I like pork dripping on pork.

And you might look at the 16 ribs in 5 hours. Not sure if that many, (in rib racks?) will be done. But you can alway bump the temp up if they don't, that's a great thing about the FE.

I usually put butts on the top rack. They just seem to cook better on top.

SmokinOkie, thats a good point about the different type rub, I do put a stronger spice flavor rub on my butts than on my ribs. I have cookded up to six racks of ribs laying flat and cook the to perfection in 5 hrs. I have never used the rib racks that I got with the FEC-100 a few months ago. Still learning the FEC-100 but so far I love it. Not sure how much time it may take to cook 16 racks, but will have to keep good notes and will know from now on.