Last year, I had a similar choice on a IAD-LAX-SYD routing. It was early in the year, and I opted for a Y ticket and used a SWU to upgrade, primarily because I wanted to make sure to accrue the EQMs for 1K status.

I still regret doing this. Not only did I end up with 125k EQMs and thus didn't need them, on the return flight there were equipment problems and I was rerouted on Qantas - and thus in Y in a middle seat as I couldn't use a SWU.

Last, the FCT in FRA is awesome - definitely worth a 4 hour layover!