Well, you can't send computers via Media Mail -- it's exclusively for books, CDs, magazines, etc. Media Mail also offers zero visibility for tracking. I use it to send books I sell on Half.com and sometimes fear they won't arrive. The USPS also reserves the right to open Media Mail packages to verify that their contents actually are books. I think it's mail fraud if you try to send other items through Media Mail.
UPS offers visibility (tracking), faster delivery, re-delivery options, etc. It's a superior service than Media Mail or parcel post.
They're really catering to different markets. I wouldn't ship a computer using the USPS, but UPS is a bit too expensive to ship books. I sent a computer once through FedEx Ground and they destroyed it (luckily they paid my damage claim). They all have their benefits. Just depends on what you're willing to pay and what services you're looking for.
Media Mail, while cheaper than Priority Mail or UPS, can take a really long time sometimes. As I understand it, the way Media Mail items are sent is similar to flying "standby;" if there's room, it goes...if not, it's held until there is room. This can create quite a delay in some cases. Priority Mail is faster, and still cheaper than UPS for smaller items (books, cds, dvds, etc).