The aircraft is assigned several days in advance, and you can check the tail number on Flight Status to see what seat you'll have. I just had a flight (738) circle through three assigned aircraft for SFO-PHX . . . the first two were the old cabins, the last and final one was NEXT, and that's what I flew. It definitely felt like winning the lottery.

To me, NEXT is definitely more comfortable, the seat has better padding than the New F. The biggest downside to me is the seatbacks actually recline a good ways (unlike the hard shell New F seats), so if the pax in front of you does full recline it cuts way into your space.