I have and use both. I, personally, use the xtreme i-30 because I don't like taking my phone out of the case. Plus, on longer runs where I am streaming satellite radio, it will hold the phone IN the Mophie Juice Pack.
My GF is using the Sport i-20 for her runs/walks with her iPhone 3g and loves it as well. Since she has an older iPhone (and it's not even a phone, more like an [mini] iPad / iPod) it works great for her.