I like the idea of the being able to pick and choose where the rails go, and the difference in cost between a modular and a fixed are not all that great.
So, could those of you with some experience with them let me know what you think of them?
modular : + flexibility (put rails where you want it)
- more scews that may come loose and get lost
fixed : + KISS (keeps things simple)
- less flexibility
Both are basically good. The choice ultimately depends on your taste / needs.