1) Go for them both, they are both fantastic games If you have to choose just pick your favorite Genre, wild west or fantasy!
What i will say is Skyrim is MASSIVE, if you havnt got loads of gaming time perhaps RD would be better.
2) A bit left field but check out the original Bioshock. Its old (and a 1st person shooter rather than 3rd), but for what you are looking for you wont beat it.
(Alternatively, if you go for Red Dead you could satisfy both to some degree.)