thanks so far for your comments
to clarify
zrh - ord (ua or co)
ord - fll (aa)
fll - zrh (co or ua)
aa is the cheapest option for the domestic segment, though I wouldn't mind taking aa all the way.
for the complete itinerary there is almost no difference in prize between co or ua, that's why I'm asking for help on quality issues of both carriers.
thanks again