This is tough. I'm 29, and Elvis and Frank Sinatra are my 2 favorite singers. Frank Sinatra & Elvis' career are much more closely related than Elvis to MJ or Elvis to the Beatles, so you question is actually the most valid comparison.
I say they are more closely related, not because their styles were the same but because their careers were similar. In the 1940's Frank Sinatra was the first teen idol, or more properly the first singer to appeal to teens. He also had a successful acting career. Sinatra was also a huge Vegas star. Superficially their styles were very far apart, but were they really. Neither Sinatra nor Presley were song writers, but they were musical icons. Both took existing songs, some popular some not so popular, and made the song theirs, to the point that the song became synonymous with their name, even if it was a big hit prior to their recording of the song.
Although I like Elvis a hair better, Sinatra had a longer career and I liked him throughout his career. Elvis' career was from 1953 - 1977 but I only liked 1953 - 1968 and 1973. Elvis made more of an impact on music, because he started everything that is today, even though the style is different it all stems from Elvis. Had Elvis never recorded, music would still be like what Frank Sinatra sung, which is fine, but what Sinatra sung was no different than what Bing Crosby sung before him, and I'm sure Bing sang like the generation before that. But no one sang like Elvis before there was an Elvis. Yea I'll stick with Elvis, sorry Frank, I will say I like your version of Love Me Tender a little bit better when you had the special for Elvis when he came back from the army, but Elvis' version of Witchcraft I liked better.
I love both, but I'm sticking with Elvis.