Just get whatever bindings you get the best deal on. If you are intermediate, and use a 5 1/2 DIN, most likely you are not going to push your equipment to failure. Specifically with these bindings they are pretty bomb proof.
I have the px12 with lifts on my powder setup, and find them to work well, and I am sure I take them places and do things beyond your skill level.
risers make getting the ski on edge easier (more of a lever to work with). So if you need that, do that. If not -- then don't. As for allowing the ski to flex easier, it's going to be a minimal difference for you.
As for your second question, railflex is nice because you can swap the bindings between skis very easily -- have them on my cheater gs skis. Otherwise compared to flat bindings, the looks (for example) -- I don't see any noticable difference in flex.. and if anything the railflex perhaps is more stiff.
good luck and enjoy the winter on your new gear