Replying to Metal rail vs pvc/hdpe

i'm expanding my setup, the situation i'm going towards is about 12ft(3.5m) height, drop down to a flat platform 3-4ft(1.20m) high

and then a second drop to zero....

now what rail should i get...

i was thinking about installing a flat down flat rail, from flat 6ft(2m)- down6ft(2m)flat10ft( 3m)

now my question is: would i have enough speed to get a metal rail or should i go for PVC ?

or do you have any other toughts about my design?

+k for constructive responses