i believe with newer lifters the trap bar deadlift is favorable because there is less back involved and the athlete can almost just stand up with the bar.
with experienced lifters they learn to pull bakc and use their own weight as leverage to get higher weights up.
this is when you take into account that the trap bar has handles which decrease the ROM of the exercise and therefore increases the weight that can be lifted (for example similar to a pinpull).
personally i hate the trap bar, it doesn't feel solid and holding my hands to the side feels extremely unnatural after having straight bar DL'd for so long.
u can push the handles down. do u think they both work the the post chain?