Thor, bless his heart, is pretty and "good" - but he's really a bit dense and has a complex. Loki is the cheeky (read Machiavellian) funny prankster/younger child, also with a complex. His complex is obviously the worse of the two, as evidenced by his significantly more retarded helmet. I can't say I blame either of them, though - Odin is a famously bad parent, who also wears a bad helmet. Thor 2 is notably better than Thor 1, just sayin'.
Loki is the only reason a lot of people watch those movies (including my dad and brother. Except my brother also watches it for Natalie Portman). Again, the sneaky funny/warped child. Not to mention he's a Woobie [of the Destroyer of Worlds variety], big time. Also, he is Tom Hiddleston.