I'd go with UA or Delta. United has always treated me so well; they are a class act. Delta is a good one to go with as well. American's always treated me well domestically, but I've never been with them internationally. I think you can jumpseat internationally, just not in the cockpit, but they may flat out not let you jumpseat internationally.
With CAL, I've never been in business. I've been on plenty of empty planes and literally gotten the last row, once a middle seat. I am thoroughly grateful for the ride, and the Continental pilots are always class act, it's just funny to see the difference between United and Continental in those regards.
Go on UAL if you can do it easily.