I have been using USPS media rate to ship comics I'm selling and trading out. Seems as though that is a fairly common practice. However, a clerk at my local post office tells me that the only difference between 1st Clss and Media is that packages shipped media are subject to search and take a relatively long time to reach their destination. The price is the same, and from the way she described it, the handling is the same, except for the search part.
Media mail is a lot cheaper - usually about half the price. From what I understand, it takes about the same shipping time as parcel post (unless it's to one of our overseas territories like Guam). But you aren't supposed to use it for anything with advertising which means you can't ship most comics.