Glass pros: pretty scratch resistant, can be cheaper depending on setup
glass cons: scratches cannot be easily buffed, weighs A LOT, not as clear as acrylic (starfire glass is clearer than regular glass, but is more expensive), not as versatile as acrylic, possibility of seam failure
acrylic pros: lightweight, clearer than glass, scratches can usually be buffed out easily usually without draining the tank, stronger than glass, versatile (can go crazy drilling CL holes etc), usually cheaper depending on setup
acrylic cons: scratches easily, if made with lower quality pieces it can yellow over time
For regular stock sizes I think glass is the way to go, but for larger custom setups I think I'd go acrylic.
-Justin