Had this dilemma a number of times. Some of the factors I consider...
- Am I in F on the original flight?
- What kind of seat can I get on the earlier flight? Can I get into F? How long is the flight? (The shorter the flight the more willing I am to take a bad seat.)
- Why is the earlier flight delayed? Weather at its origin that will not affect the remainder of the trip? ATC delay at the destination that may end up affecting your flight too? Mechanical delay? Check OIS and the flight status notes on ExpertFlyer. (And never switch to a flight with a mechanical delay unless the problem has just been cleared and they are about to close the door.)
- Probability that my original flight will leave on time? Is it at the gate or in the air with an ETA that will allow an adequate turn time? (AA is notorious for posting flights as on-time when it's pretty obvious they won't be.)
- Will I be able to get on the earlier flight early enough to stow a bag? (I got onto an earlier DFW-AUS flight only to be told at boarding that the bins were full and I'd have to check my bag, which added delays at the other end that almost fully negated my time savings.)