I wont go into technobabble as you can find all the info on the titleist website but for me the difference is that the nxt is slightly softer than the pts solo, so there is a little more spin and control on the ball. Without running the risk of when you hit the odd bad shot of cutting the ball in half like the old ballatas.
Hope that helps.
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Titleist 983k - fujikura stiff shaft
Taylor Made R7 Steel 15* - Re*ax stiff shaft
Mizuno MP32s - S300
Mizuno MP T Raw Haze 51*, 56*, 60*- S300
Scotty Cameron Red X - 330g