A V10 or V12 simply has more combustion happening per revolution of the engine making the engine "smoother", that being about the biggest difference. Lots of things come into play, and I'm not an engineer. Typically most people when they think of a V12 would think of something like a Ferrari where there is so much engineering that goes into it that its hard to compare it. Some may be interested to know that GM proposed creating a V10 version of the LS series of engines back during development, here is an old article that is somewhat relative to this topic and if anythign you may find an interesting read.
http://www.gmhightechperformance.com/tech/0512htp_gm_stillborn_gen_3_v10_history/index.html