my 0,02 ...
I'm in the process to set up my alarm system by connecting all sensor to a Weeder I/O board.
They are 14 channel digital I/O boards, driven via RS232 and stackable also. They are also cheap (they cost 59$ each + shipping), surely cheaper compared to GC100.
I assume that when you say 20 wired zones you refer to 20 sensors (door/window sensors and similar), and this leads to the fact that you need 20 digital input channels.
If all of your wiring converge to a single point (as in my case) you may find Weeder boards very cost effective. If not, probably you better stick with GC100 as they offer an ethernet interface that make them easily to distribute around the house.
No real idea for wireless zones, maybe some one else can give a hint on this.