Although I’m biased (I’m going to L’ville next year), I’d say Lawrenceville.
First of all, the school has an amazing location. Within a 10 mile radius of the school, you can go to downtown Princeton, 2 malls (one with an Apple store), and the school allows you to go to certain shops in the village of Lawrenceville without needing permission to leave campus. NYC and Philly are both right near by, so it isn’t hard to fly into the area, and it’s also not located in ‘the middle of nowhere’ like Deerfield has been said to be. Also, the weather is warmer 
I’ve also found Lawrenceville to be a quite humble school. When I was originally planning on visiting, I was worried that it would be full of insufferable snobs, but my tour and interview completely changed that. It’s quite important, in my opinion, to be aware of the community around you, and Lawrenceville cultivates that.
I also liked the ‘house system’ at Lawrenceville. Since the houses are smaller, you can cultivate a closer relationship with your peers that live there. Even day students (like me) are included in the House system, and it helps foster a great community.
However, if you get a gut feeling that Deerfield is better for YOU (don’t base your decision on prestige), go for it!