First thing I want to do is state that I am not proposing a trade, or that the Twins or Angels would consider this. This is just a thought experiment to provide some entertainment as we suffer through a day without baseball.
So if you could have either player, but not both, do you want Joe Mauer or Mike Trout? And why?
With Trout you get an 18 year old with unlimited potential, with the stats to stand out in the low minors and the tools to make the scouts drool. Once he's ready for the majors, he will be cheap (by baseball standards) for his first 6 years. But be warned, the development of prospects is an uncertain game, as Angel fans know all too well.
With Mauer, you get a guy with the production and tools in the prime of his career. He's 27 years old and on a path for the Hall of Fame. The only problem, Joe is expensive. For the next 8 years he will make more than 20 million per season. That's 20 million dollars you could spend on other players if you had a cheap young star instead.
With both players, you get quality guys with no baggage who are dedicated to the game. Who do you want?