I've done both and would say reading is better if you held a gun to my head, but both worth a jolly. I'd base your your chose on which is better for you logistically if it's your first one you might to think about getting home after for ever the tube or driving. There are a lot of races down there though I'd see what else is on you might like before entering. Northern monkey so got limited knowledge for London village.
If you feel really tired and find yourself slowing down to a crawl.....then don't give up....go and have a 20 minute sleep.( set the alarm).....its amazing how much difference that catnap can make....you will be up and ready to go.....don't have much longer because if you fall into deep sleep you will wake up after a couple of hours feeling crap....
I would make sure you have energy/food on standby well in advance and to hand if needed.