Speaking as someone who lives close by, here are a few pointers:
Falmer is basically a train station with two universities either side. There is pretty much nothing else there. The trains on this route are often as small as two carriages, and the service is at its weakest on Sundays. The station is far too small to accommodate a matchday crowd and queuing for more than an hour waiting for a train is quite common. A clever / sneaky move is to park on the campus at Sussex University, but getting out afterwards can be a pain because the junction is not set up to deal with any significant volume of traffic. There's an ok student bar on Sussex campus (north side of the railway line) but the one on the Brighton University campus (same side of the railway line as the stadium) is rubbish.
Brighton is a VERY expensive city. Buses are overpriced, and cabs from the station, whilst an option, will empty your wallet. As for pubs.... yes, there are lots to choose from. By and large however, they are nothing special and eye-wateringly expensive. Most are quite cramped too.
Lewes is a much smaller town, but the pubs are generally much better quality. Not really any cheaper, but at least you get better value for your money. Lewes is also a better-run train station than Brighton, and the trip from Lewes to Falmer is shorter / quicker than the trip from Brighton to Falmer. The Brighton station staff are notorious for herding people like cattle and then all vanishing from the concourse the moment anything goes wrong.
Basically, the longer you stay at the stadium after the game the better. The post-match travel is appalling, because the site simply does not have adequate transport links. You might not even be allowed to use the pedestrian bridge in Falmer station to cross over the railway line without queuing first, so I would settle in at one of the stadium bars and wait for the crowds to disperse.
