Song of the Year is a songwriter's award - it's specifically for, well, songcraft. It is awarded to the songwriter(s), not the artist. Unless, of course, the artist is also one of the songwriters.
Record of the Year is, supposedly, more about the overall production of the track and is awarded to the artist, producer, engineer.
And, yes, nice to see Alt-J recognized. I don't know about St. Vincent. Were Alt-J to lose to anyone else in that category, I'd imagine it would be Arcade Fire. That being said, that kind of category's tricky, because a Grammy voter has to have chosen it as one of his/her specialty categories to vote for, so one has to assume s/he has an interest in alternative/indie music. So, sometimes very big names - in this case, Arcade Fire - can lose to lesser known artists. But, frankly, Alt-J have been having a very good year, so they're hardly a small name, either. I have to say, I'm surprised that Arcade Fire didn't get any of the big categories given that they've won album of the year in the past.
although, that year, a lot of ppl were asking "Who the hell is Arcade Fire?"