In one, you are specifying a hard pointer to a file. In the other (preferred) you are specifying the URI of the TLD which can be anywhere in the folder hierarchy under WEB-INF.
ok, now it' clear The problem borned when using myEclipse it give me an error of sintax using the uri generic declaration when i was not connected to the net.. probably it was an ide error if you say that it search the tag in all web-inf dir.