OP I was where you were a few days ago and there are some pangs of regret when I see the 60" P50 is $300 less than the 55" UT I bought, because I don't know that I will use the smart stuff either (and deciding how much, if any, I will invest in 3D now) but there are some things to consider.
Look at the handy comparison chart
Panasonic provides on their site and in the Amazon sale descriptions. The sets look similar, but I see no DLNA or Focused Field Drive on the P50 and believe I will be grateful for those.
I also begin to wonder what kind of components are in the P50 to make it so cheap. Read the P50 and UT50 owner threads and decide yourself. That's what persuaded me, plus there was fewer complaints about input lag from gamers when i searched the UT50, but that's purely anecdotal. I got it in my head the P50 was bad for gaming.