Is it a better idea to buy a home shredder? Or use a service?

I’m sorting, scanning, and purging old documents for my mom’s law practice. She’s having me look into how to destroy the already massive pile of papers that have accumulated. Here’s a picture of what I’m dealing with so far—edited for security.
http://oi62.tinypic.com/2dhen2a.jpg

I’ve contacted several shredding services for quotes, but no one has gotten back to me thus far. I’ve also done some research on home shredders and it looks like Staples is having a sale.

From what I’ve heard, home shredders are fickle, but would using one be cheaper than calling a service? This pile isn’t even half of what is to come when this project is finished.

EDIT: I live in NYC, if this helps with suggestions.