Letterman's projects to be better at this point for two reasons:

1) David Letterman actually interviews his guests. He's not scared to ask a tough question, and when he does, he usually does so in a hilarious manner.

2) Letterman makes his audience feel like he and they are in on a joke, sometimes at the expense of the guest. That is an awesome ability that I do not think Fallon will ever develop. Fallon is in on the joke with the guest, and sometimes so at the audience's expense.

The Joaquin Phoenix or Paris Hilton Letterman interviews are both good examples. They both seemed so uncomfortable but it was so hilarious.