Bruno Mars was good the year he did the half time show, Beyonce too. But I can't think of anything recent (like within the last 6 month) that either of them have put out. No new jams since the last time they each performed, so seems like they're just trying to bring out the performers that received positive reviews in the past.
If they could get Lady Gaga to do the anthem, why not the half time show? I think she would have been a better choice. Another rumor that was floated for a while before they announced it was Coldplay was Metallica. I'm not a huge Metallica fan but they are from the San Francisco area, and would have provided more energy than Coldplay.