I've spent time with both of these frameworks and the answer is "it depends".

I'd recommend Play! over Lift if:

  * You just want a data entry / CRUD style app
  * You prefer MVC architecture
      - Play! will feel pretty familiar if you've used Rails
  * You don't know Scala

I'd recommend Lift over Play! if:

  * You already know Scala
  * You want to build more "reactive" apps 
     - Lift has excellent AJAX / Comet support built in
  * You enjoy trying new ways of doing things
     - Lift has many unique features, there aren't many similar frameworks

They're both fast, have excellent communities, decent documentation (I think Play's documentation is better today, but Lift's has been improving fairly rapidly). You can't go wrong with either one.

I should add - you can do AJAX & Comet in Play!. You can do MVC in Lift! One framework is just more adept than the other by default in their respective arenas.