This patent says the former causes diarrhea while the latter has no GI impact. Rats were used to see how far the charcoal advanced along with the l_carnitine.
I have both in my kitchen, should I toss the tartrate ?
Description of the problem, the lab rat tests and resulting patent :
http://www.patentstorm.us/patents/6441039/description.html
Video tartrate from manufacturer :
http://www.5min.com/Video/What-Are-the-Benefits-of-L-carnitine-Fumarate-213399578