Two professors want me to work at their labs, and I'd rather work with one of them than the other I'm an undergrad who wants to do research but I don't know many professors, so I asked my school's Dean for Undergraduate Research for help. He told two professors about me, and both showed interest in having me in their labs.

So my problem is, I only have time to work for one of them, and I'd rather do research with Prof.A than Prof.B because Prof.A is more senior/experienced professor, and just seems to be a much more interesting and approachable person than Prof.B. However, they share almost exactly the same  research area/interests, so I can't really say "I prefer working with Prof.A because his research interests align better with mine". But of course I'm not going to say to Prof.B's face "I think I'll have a better time working with Prof.A" either...

How should I politely/gracefully let them know of my decision?
I don't think you have any obligation to explain the reason for your decision.  Just tell Prof B that after consideration, you decided to work with Prof A, and thanks for his interest.