Honestly on the wall you'll get way more dust and hair than under the desk. The biggest problem with being under the desk is you might hit it with your legs, kids might get to it (if you have kids), and animals will be curious if you have those too.
I do have a kid who stays away from the desk at the moment but will be very interested and will try to get in at it so that is the biggest issue with under desk.
If i can get the wife onboard I will go for the wall mounting. I will change my motherboard for something more visually pleasing and with the ability to SLI later if I get the go ahead.
Thanks, as always