My son has tried both. He first tried Bentyl but Levsin worked better-my GI says that if one doesn't work, try the other. Neither one was very effective, though, so now he is on Donnatal, which is a cocktail drug and works a little better.
Elavil was amazing at first-the only thing that ever had a strong effect on him. Suddenly for the first time in his life, he could fall asleep before midnight. But it only lasted 2-4 weeks. However, I've read that tricyclics can have a constipating effect, and as that is an issue for us, it didn't seem ideal. His current psychiatrist switched him to Zoloft--suposedly the SSRI's have more of a loosening effect.