Based on our experience firing about 100, 15/16" dia. steel ball bearings, lots and lots of cast lead shot of .985" dia. and 20 cast zinc shot of .960" dia. we can safely tell you we WILL NOT be firing more steel ball bearings. They are far too dangerous because of energetic bounce-back. Lead shot is by far the easiest to produce in quantity and zinc would come in second for being accurate and less weight than lead, but slightly more hassle to cast.
As far as marbles go, we would not fire them or suggest they be fired, because there are more you-tube idiots out there than you know. They would want to start with 300 or 400 grains of FFFg BP rather than some historical pistol load. Firing excessive loads with glass marbles is asking for a snoot full of powdered glass. NO thanks, we will leave marbles to others. If you do find yourself firing them, be sure the the wind is at your back so the the glass powder will blow away and NOT into your lungs!
We have 7, 1" bore dia. cannons now and it is a great size with shot or bolts in rifled cannon. Lots of fun can be had with this size cannon and without huge expenditure of powder.
Mike and Tracy