Brighton or London? I think you're lucky if you get the choice; it's hard to get a place for London. I'd enter both and if you get two places decide which one you fancy, then roll the other one over to the following year. Both races allow you to defer running, if you notify the organisers in time.
As for which one is "best", depends on what you are looking for. London is an amazing carnival of running, but very crowded if you are running in one of the popular time slots. That might be frustrating if you want a clear run at a PB. Then again, it's no more crowded than some of the bottlenecks at Brighton.
Forgetting about time, and looking at the emotional scale:
London - I've done this three times. Always experienced: tears and photos at expo when collecting race number, hero's send-off the night before, excited at start line, high-5ing supporters for 20 miles, screaming at the crowds for the last five, choking up on the Mall, tears on the finish line.
Brighton: sense of quiet satisfaction at beating sub-4, at last.
New York: my first marathon, and only my second ever race. Off the scale. Life changing experience. (and note: a charity fundraising target lower than for London, plus long training runs in September, not February)
I know these big marathons can get a lot criticism sometimes, but I just read this on the BM website and it really made me smile. We were all part of something very special....
The city's hospitals announced there were fewer patients than on a normal Sunday. Statistics produced by St John Ambulance indicated that of the nine people taken to hospital only three were marathon related, an extraordinary figure.
On the crime front, there was less crime, the reason being the camaraderie created by the running fraternity. Violence and drunkenness disappeared from the streets as runners high took over not only competitors but spectators as well. According to sources it was a crime-neutral event, which is as good as it gets.
People came out of their houses at points where they were not expected, for example from Grand Avenue to King Alfred's Sports Centre. The only explanation for the swarms urging the runners on was that the city was infected by marathon fever.
There were nil traffic problems, partly because many streets were closed, but many motorists either stayed put and used their feet or left the city.