[LONGHAIR]

They are deffinately different.
The sender for the gauge should be progressive...making it rise slowly.
The one for the idiot light will jump suddenly.
Why not just test it?
The sender is just a "grounding" unit. If you ground your wire to the gauge it should "peg" the gauge. The gauge sender works like a rheostat, progressively giving more ground. The idiot light just trips all at once, when it hits a pre-determined temp.

If you heat the gauge manually (gently with a propane torch or something similar) while keeping the external housing grounded, you should be able to tell which way it works. The pain factor in this is the you have to remove it for the test.

Another thing that can affect the sender is the ground itself. If you have liquid pipe-dope or excessive wraps of tape........it may not be getting enough ground to go any higher. Since you said that is goes up some and stops, this may be your only problem. If so, you still need to take it out to fix it..........might as well test it while it's out.