There are two big differences. The first has longer term impact, because it's basically who runs them. National Parks are run by the Parks service, natch, while National Monuments are run by various and sundry other federal services ranging from US Fish and Wildlife to the Department of Defense (Which is think is just White Sands, but I could be wrong).

The other, and highly relevant one right now is that, due to the wording of the Antiquities Act, Congress creates National Parks, but the President has "nearly-unfettered discretion as to the nation of the object to be protected and the size of the area reserved"<1>.

This means that Mr Obama doesn't need to get a hostile Congress in line in order to designate the area a National Monument, but would in order to get it made a National Park.

>Gatlinburg, Tennessee

I can't read this and *not* think of A Boy Named Sue.
--
-Pasha <1> Wikipedia
"Don't change the subject"
"Too slow, already did."