I used foam for most of my armor and have been replacing it a little at a time. It has it's advantages (light weight, cheap) but isn't durable at all. I spend several hours after every convention repairing and repainting the foam due to the wear. I'm not sure what you mean by "dissolves", but I haven't experienced anything like that. The problem with rigid armor is it tends to rub the paint off the skins where the edges meet the latex. For example, My chest armor is a rigid shell covered in sintra and foam, and there's always allot of wear on the chest and abdomen area to repaint after a few hours wear. All foam would seem to be better, but the foam can't take the flex, leaving you with cracks and peeling paint to repair. In my opinion, latex, either cast into the skins or worn separately is the way to go. What I've learned and didn't think about before finishing the suit was that no matter what you do there will always be repairs needed after extended wear of the suit. If you have the time and resources I would stick with all latex.
Brian