Unfortunately, we don’t have much experience with machining plastics. Of the plastics we can cut, we have heard that PETG, Styrene, and ABS are easier to machine than acrylic. We can get PETG in clear, styrene in white or black, and ABS in white or black with one side smooth and the other side with a haircell texture. Here is what McMaster-Carr says about those plastics at http://www.mcmaster.com/#standard-plastic-sheets/=hc420l:
From what I can remember, 1/8" PETG, styrene, and ABS are not noticeably flimsier than polycarbonate, but if flimsiness is a big concern, please try purchasing samples from a supplier such as McMaster-Carr.
Please note that we normally get these plastics in the 3mm thickness (0.118" plus or minus 10%) rather than 1/8". We might be able to special-order 1/8" if 3mm will not work for you. Also, the laser-cut edges of PETG, white styrene, and white ABS tend to have a dark-yellow tint to them that might be hard to clean off. PETG usually comes with a protective plastic film that tends to peel off and tear easily so it doesn’t completely protect the sheet from scratches, and styrene and ABS typically do not come with any protective film. If you end up ordering PETG from McMaster-Carr, you can make special requests to make sure they pick out a nice sheet with good protective-film coverage.
I’ve played with polycarb, acrylic and recently some PETG. PETG does machine (scroll saw, drill press) very similarly to polycarb, but it isn’t quite as tough, and cuts a little faster. It wont splinter and shatter like acrylic. It will bend very far and still spring back, and when you do exceed is elasticity, it becomes plastic, instead of breaking/snapping.
PETG feels more rubbery than Polycarbonate in general. I’m not exactly sure just how much less stiff a 5"x10" piece would be though. It wouldn’t be a big difference, and the actual characteristics of how they bend will probably vary more than a direct measurement of stiffness. I agree with candice, getting some sample material would be a good idea.
It does machine very nicely though, so that shouldn’t be an issue at all.