They both have their advantages:

• The Graphic Link uses the least amount of characters on your site.  The graphic is coming from a very reliable server with a 99.9% uptime.  You should rarely see that annoying little red "x" that means a graphic is unavailable for download.
• The Alternative Text Link uses a few more characters, but completely eliminates the possibility of the graphic being unavailable for download.

Either way, it's your choice as to which link you would like to use.