I think the question you are asking is more like "where do you prefer to have your programmable keys?"
I have to be honest here. I've tried to use the extra button on my G5 mouse, and I've tried to use the special keys on my G15 keyboard.
The only thing that has ever worked for me is a readily available normal key, like the tilde ~ .
So really I like simplicity. My next keyboard will be something like this:http://luxeed.com/estore/product.php?id_product=43