Originally Posted by Santander

If all you want to do is earn and burn, I'd put it in US. There's ample opportunity to buy miles on the cheap or earn through other methods like Grand Slam (even if you don't live in the US, I don't) and the reward chart is quite reasonable. No surcharges for redemption either. UA is also good for redemption but there isn't as much opportunity for boosting your mileage as US.