As you can see from the above comments, both carriers have their fans and detractors. I've had good and bad experiences on both (within the US). Either of the mainline carriers would be a better choice than the other's commuter carrier, if that makes any difference. From your comments, I'm inferring that both ORD and FLL are planned stops, not just point in a mileage run or to get a specific price. If that's not the case, others may have additional suggestions for carriers to consider.

To stray a little from initial question (apologies if you've already considered these): (a) unless you are on the non-stop LX (code share AA?) or direct UA flight ZRH-ORD, you will probably have to change planes en route to ORD; thus you should consider whether flying into (or out of) MWD instead of ORD is an option and whether gives you better alternatives; (b) from your description, it seems as if you are describing the trip as an international open jaw and a domestic one-way; if price is critical, you might try pricing as 2 round trips (for ex. ZRH-ORD-ZRH and ORD/MDW-FLL/MIA/PBI-ORD/MDW); (c) PBI and MIA are fairly close to FLL; MCO is only about an hour away by plane and often has very competitive prices since a lot of carriers are taking a lot of people to Disney and other nearby attractions; you can probably find MCO-FLL for under $100. Consider checking into these airports as alternatives.

Good luck.