I thought I'd ask the experts here what rationale they use to decide whether to switch FF programs from one domestic carrier to another. When have you made the switch, and was it a good idea or a bad idea?

I've accumulated silver status and about 70,000 miles on United, but the lure of Continental -- plenty of flights from Newark to Europe and Newark to LA/SF (my chief interest), as well as the unlimited space-available domestic upgrades, AND newer planes on the routes I fly -- has me thinking that with the start of the new year, I might as well switch carriers. On the other hand, United has a much better international alliance, and I've already earned a bunch of miles on United and have a MP credit card.

Any advice in this particular case?

Also, any good stories about the good and the bad of switching programs?