I prefer the quads because I have <25 MAC powder shadows and don't plan on building a large collection. I have one premade quad (Nat. Eccentric) and 4 others that are grouped according to color (brownish neutrals, pinks, purples and misc). They're easy to carry around, regroup according to whim and store in the small 3-drawer storage unit I keep my eye products in.
I keep Night Owl, Taupeless and Slip Pink in their pots just because I'm a little anal about their LE-ness.
But the latter two may get depotted because I use them frequently and want them more accessible.
If I had lots of shadows or planned on a large collection, I'd do 15 pans with a couple of 4s for travel for the same reasons many posters have already outlined.
Enjoy your palette-building, whichever size you choose!
merleskaya