Brighton or Dover

I love Dover castle. I was underwhelmed by Brighton.

I would say the difference is personal preferences.
Dover can however be combined with Canterbury on a day trip.

Brighton would get my vote, particularly in early March.

Depends what you want.

Dover Castle is outstanding, one of the finest castles in England. But the town really isn’t much to write home about.

Brighton is a fun town with a disappointingly stony beach but a brash seaside vibe and some good hipster shopping in the Lanes, plus one of the most extraordinary royal palaces there is - the Brighton Pavilion.

I'd be inclined to opt for Brighton although I'd probably make the effort just for Dover castle.

Dover Castle is magnificent, my wife and I visited in 2000. We did not visit the town but my research suggests it's nothing special. We drove through Brighton on that trip and I had the chance to gaze longingly at the Royal Pavilion on the way to dropping off the rental car. That's about as close as we got, but it seems like a really great place to visit. Based upon research and not personal experience, I think Canterbury deserves consideration too. Full disclosure, I am not generally a big fan of day trips from expensive big cities. You pay a lot to stay in London, and trains are expensive there too. One consideration might be, how close are you staying to the train station that serves your destination. It can take a long time to get across town to the station where your train leaves from. (I am only assuming you are taking the train, as that would be the practical option).

I agree with vftravels' philosophical points, though I enjoyed Brighton (and Canterbury).