I think you'll be happy with either yellow or red. I also was trying to decide between yellow and red, decided on yellow, but ended up with red because I was too impatient to wait for yellow to become available. But the red is great, I love it! And I'm sure I would have liked the yellow as well.
Choose based on what color you like better in general, not in terms of how it looks on the phone (because they both look great on the phone)! If you were to rank colors would you rank yellow higher than red, or red higher than yellow?