Ollie,
I think it depends to some extent on what kind of functions you are doing.
You mentioned schools.
Darren is obviously right about a hazer being more subtle, but I can't help thinking a smoke machine would go down better with school kids...purely because it's NOT subtle!
The last time I used my smoke machine was a few years ago at a fourteenth birthday party, and they wanted loads of smoke. The more the better, in fact. If I'd had a hazer, I'm pretty sure they wouldn't have been very impressed.