Both are great and you will have alot of fun with both, depends on what you want out of a game though. Red Dead is awesome, the campaign is amazing and its side missions will keep you busy for a good 30 hours and all of it will be epic, on top of that you have a pretty good online system, with free roam with loads of friends and deathmatch type stuff as well.
Just Cause is more over the top. The world is huge, and the 'campaign' is pretty entertaining. There is hundreds of hours of game play if you wanna do everything, and you definitely will get your moneys worth.
Both are fantastic, and you really can't go wrong with either, but I would go with Red Dead, its one of the best games I have ever played, its only flaws are the numerous bugs that plague it, but they get fixed in time. I would rather have 30 amazing hours, than 100 pretty good ones.