There's upsides and downsides to both games. For Skyrim, the game will likely take longer to do all the quests and could last you 100+ hours. However, if you're playing on the PS3 (which I assume you are since this is the PS3 forum), you will have to wait 30 days longer than on the Xbox 360 to buy any DLC that comes out. Also, with most recent Bethesda games, it will potentially be very buggy, as mentioned above.

In Rage's case, it has a huge mandatory install on the PS3 (somewhere around 8 GB I think.) This means not only will you need a large chunk of free HDD space, it will also take an extremely long time before you can even start your game. Other than that, I can't think of anything else negative about Rage, and I'm not all that familiar with id's games so I don't know if they usually have problems with bugs or not, but they did push the release date back a month, so hopefully there won't be too many.

In my personal opinion, I plan on picking up Skyrim this holiday because I think it looks fantastic and fixes all my problems I had with Oblivion. Rage doesn't really appeal to me and that massive harddrive install makes me even more weary. But, barring any bugs, installs, or settings, I would advise you to go with the one you feel you'll enjoy more, whether that's a 15-20 hour fps or a 100+ hour RPG is up to you.