Cool tube

I wana buy a cool tube, but i don't know what difference makes the 125mm comparing to 150mm. It's the diameter? How i discover which one i need? The 125mm or the 150mm...
Also, is there necessary a little vent at the bottom of cool tube? Or is just if you want to attach or not...

Thanks all

yes its the diameter of the cooltube , 125 mm and 150 mm are up to you but if you are using a 400w or 600w the 125 mm works. and ofcourse an inline fan with matching diameter is a good thing =)