While I agree that commuting from home may be cheaper (especially if you live near a train station), it's also important to consider the travel time to the San Diego Convention Center itself--including the train ride and the shuttle and/or trolley ride. Based on my own estimate from using the Amtrak trains and buses/trolleys on any given day of the week, it could take anywhere between one to two hours or more. Like what kvthai mentioned, the trains also have different departure times than what you expect.
The closest train station to the convention center is located at this address:
1050 Kettner Boulevard
San Diego, CA92101
(On Amtrak.com, it's listed with the code "SAN.")
Whatever the individual case may be, it's always a good idea to be prepared and do research ahead of time, so good luck!