I don't suppose you're looking for an answer such as, "I like 'em both..."
Choosing one of these two is extremely difficult: both have lots of talent, both are similar hitters, both are red-hot. At the moment, Simon's stats look a little better (in spite of Cox' .370 average) - he's showing a bit more power, and has only struck out once. Still, it's too early to rely on stats alone.
My choice would be Cox, for just one reason: when these guys have slumps (and at their level of experience, they will), the Rays will show more patience with Cox than Detroit with Simon. Cox' name will appear in the box score all season, but the Tigers have lots of options at dh, especially since they're now planning on using one primary catcher. I don't think anybody can accurately forecast which of these guys will finish the season with more hr or a higher average, but the odds are in favor of Cox playing more games. Go with him.