If you value your time, you may prefer to fly Southwest (assuming that is what you mean by "JetCheap") MDW-OAK instead of United or American ORD-SFO simply because MDW, OAK, and Southwest have far better on-time records than ORD, SFO, United, and American.

On the other hand, Southwest's Rapid Rewards program is not particularly good for passengers flying longer flights (it gives one point per flight, regardless of length, with 16 points within 2 years to get a free but capacity controlled award).

In terms of rules of thumb, be aware of alliances and mileage partnerships, so that you can concentrate mileage as much as you can instead of scattering it.

For example, if you have a United account, you can credit mileage from US Airways, Air Canada, Lufthansa, Singapore, and some other airlines to your United account.