Technically RE5 is an easy winner, also it follows the "more interesting" Wesker storyline unlike RE4 which is considerable as a sidetrack. I really appreciated the realistic weaponry of RE5.
BUT in horror survival aspect RE4 shows superior quality; better locations, dark and scary atmosphere (Africa in RE5 just doesn't fit).
I finished RE4 about 10 times on PC, 3 times on X360, as for RE5 about 5 times on X360.
So my vote goes to RE4 .
I must add: Chris is a badass. CHRRIIIIIS!!