I think probably the biggest factor is the opening that you tie together. The foil balloons are harder to close. I believe in the stores where they blow up the foil balloons they have some special way of closing them.

I suggest the first thing you should check is the way the balloons are closed. The rubber ones are usually easier to tie really tight.

Interesting observation!

Obviously, the foil balloon was losing air more rapidly than the rubber balloon. Either the rubber is actually less permeable to air, or else there is a leak in the foil balloon. How was your foil balloon constructed?