In my opinion Splinter Cell is better but you should really look at the reviews since Splinter Cell and Rainbow Six have almost nothing in common except the Tom Clancy name.
Are some sites to go to for reviews with screenshots:
Rainbow Six 3
http://www.gamespot.com/pc/action/tomclancysrainbowsix3rs/index.html
http://pc.ign.com/articles/390/390224p1.html
Splinter Cell
http://www.gamespot.com/pc/action/splintercell/index.html
http://pc.ign.com/articles/386/386386p1.html
Are some sites to go to for reviews with screenshots:
Rainbow Six 3
http://www.gamespot.com/pc/action/tomclancysrainbowsix3rs/index.html
http://pc.ign.com/articles/390/390224p1.html
Splinter Cell
http://www.gamespot.com/pc/action/splintercell/index.html
http://pc.ign.com/articles/386/386386p1.html