Thanks for all your input. I was already leaning towards United, specifically because they serve my most frequent destination, SFO, which costs just one upgrade coupon. Even though Delta has more non-stops to the east coast and has even upgraded me on L class fares (twice!!), United offers more and better connection times overall. And yes, their biz and 1st class sections are nicer. Still, I'll probably fly Delta to Prague next fall but I'll definitely apply the miles to United.

Again, thanks for all your thoughts.