i have done both. they are both equally as sturdy, it just matters how you build it. i find that putting the pvc on last and screwing the the top is the easiest to do; however, there is bigger chance of catching and edge or breaking a pvc pipe. to be honest it doesn't make much of a difference, i rarely catch edges due the the holes (if ever). most of the time i catch edges on pvc is just well normal edge catching (if there is such a thing) i just built a new segment for my rail out of some old pvc and scrap wood i had. made it real quick in like and hour and a half (quick for me) and screwed from the top because i think it is easier to put together that way. (i have 4 segments of rail total, 2 screwed from the top and 2 screwed from the bottom)
Sparknotes: doesn't really matter which you do. i would screw from the bottom, maybe a little better.