In general, Reverse grip exercises require less weight to go to failure, than the normal grip variation using the same number of reps.
Put it like this. If your limit on standing E-Z Bar Curls was 15kg's, (33lbs), you use a Reverse Grip and you'll be screwed in seconds. Assuming that is you could get beyond the first rep.
You'd be looking at approximately around 5kg's, (11lbs), less weight on a Reverse variation.
You can however sometimes use the two differing positions in the same exercise, I.E. Zottman Curls, where you Supinate going up and Pronate going down.
As an example of the difference between the two grips. Hold one closed fist around head height. palm facing away, then with the other hand, lightly grip the Forearm near the Elbow and rotate the closed fist so it faces you.
Then do the exact opposite movement, back to the start point.
Doing this a few times, you should feel a pulsating or flexing type sensation in the medial, (inner part), of the Forearm area, if you're doing it right. This will emphasise the difference in potential stimulus to the area, depending on the type of grip used for a given exercise, as not all the muscle and connective tissue will be in the same position, depending on the grip used. You'll start with a difference.
Hopefully that makes sense