What I gather from reading the forums here is that AC and NH are pretty good about letting their members have access to award seats on other carriers. United sometimes (routinely?) blocks seats. I don't know about Continental. If you transfer your Amex points to CO, eventually they will become UA miles because of the merger, but what the new FF program will look like is anyone's guess (including whether it will be easier or more difficult to get *A award seats).

When faced with a similar problem with AMEX points, I opted to put them in CO's program because I have a lot of UA miles. But if you don't already have miles (or the opportunity to earn miles) in a FF program you can transfer the points into, then you should spend some time browsing and asking in the individual airline forums to get an idea of how easy it is to redeem the miles.

(By the way, this seems like a great question for Lucky - click the "Ask Lucky" link on the right.)