I’m a current Lawrenceville student who’s visited Choate, and I’ll say this about the campuses: Choate’s is a lot more spread out than Lawrenceville’s, and it has some actual roads going through (as cityran said, Lawrenceville is quite self-contained, besides Main Street being right across from the school). Even then, if you’re walking to class in the morning, you’ll usually have to watch out for cars since day parents will be dropping students off, but during most of the day you don’t need to really pay attention. Some other nice things about Lawrenceville’s location are that it’s a.) very safe and b.) has many nice areas nearby—Princeton and Lambertville within 20 minutes, and NYC and Philadelphia within an hour. Both schools have a plethora of historic architecture, ugly 50’s-70’s buildings, and newer, more modernistic buildings, although I was definitely impressed by Choate’s “Colony Hall”—it looked twice as big as our KAC, and that was on top of their already existent theatre center!
I obviously can’t comment on the vibe in Choate’s school community, but as my other answers about this topic have said, the house system is a huge factor in Lawrenceville’s social life—both day students and boarders are fully integrated into it, and you will end up having your best friends within your house. It also results in some playful rivalries :). Lastly, the Saturday classes definitely help make Saturday nights more lively (speaking from personal experience, they’re my main excuse to stay there throughout the day), since pretty much everyone is around, and with unique facilities like the Ropes Course there’s a lot of activities that can be done.
Hope these answers helped—if you have any specific questions, let me know.