The muffin pans are real hit or miss. I have some that work beautifully, but I have others that were absolute disasters. The aluminum ones get really soft when heated by the molten alloy and just collapse, and the ones that have the cups soldered to the surface tray separate just like you experienced. What you want are the old one piece stamped steel ones, or cast iron. The muffin ingots also don't stack well. I have 5 gallon buckets that I fill about halfway, so I don't worry about stacking. It would be a heck of a lot more convenient if I had an ingot shape that lent itself well to stacking though.
That said, I think the welded angle-iron moulds are the real deal. They're darn near indestructable, and if you leave them outside in the rain so they rust a little bit, they'll drop the ingots with no effort on your part. It really helps if the ends are angled out slightly though. Ends that are perfectly square absolutely don't help ingot release at all. I should point out that I don't have any angle-iron ingot moulds though, but I've read quite a bit in preparation to have a friend weld some up for me.
If the angle-iron moulds sound like something you want to pursue, you might do some searches on
http://castboolits.gunloads.com since there are quite a large number of threads including good photos there.
Mike