overflow questions

[roverT]

Hi, I have a 135 gallon reef tank that is currently running. I also have a 30 gallon sump that I've had for some time that is not running. The 135 is not drilled or anything. I would also like to get an overflow. My questions are how do overflows work and which one would be good for me(I've been looking at the C-siphon overflows CS-102 or the 150)? Oh yeah I only have about 4" of room from the back of my tank to the wall but I could put it on the side of my tank.

Thanx
Trevor