MicroMister nailed it. John - you must be a fellow plastics geek to rattle off all that crap..... (You gotta be one to spot one!)
As far as weight - there is not enough of a difference to worry about. It's close enough to one another.
Plexiglas is acrylic and is darn tough stuff. It does bond well with CA. If you can cut with abrasive cutting methods, you are probably better off. Scoring and breaking will most likely just lead to blood loss.
Polycarbonate is great stuff. I.e. better stuff!! As I said, plexi is tough, Polycarb is incredibly tough stuff. It is very machinable. It can be drilled and tapped. (Do NOT use any cutting fluids - it will craze). Epoxy sticks well. If you can get your hands on MEEK - it is a solvent that is the absolute best for Polycarb to Polycarb bonding. It is not a "glue". It bonds at the molecular level. It is very NASTY stuff however!!!
The down side of PC is crack propagation. Once a "sharp" forms - it will propagate the sharp as a crack. If you drill holes, counterbore both sides of the hole with a 45-degree chamfer. It you get a knick somewhere (watch those leading edges), round file or sand out the nick so it is nice and "smooth". If you get a crack that starts to form, you can head off the crack by drilling holes at each end of the crack and putting a chamfer on the hole(s). (yes - drill right through the fin at the end of the crack.. You can fill the hole with epoxy if you want - but the drilled hole will stop the crack from continuing.)
Other sources to get PC sheet that I see mentioned here all the time is McMaster Carr.
www.mcmaster.com They have good thickness selection and sheet size. They also sell black, but only in 1/2" or thicker. Black is much more UV resistant. UV shouldn't be a problem for most rockets, but PC will yellow over time with sun exposure and that will lead to a lower impact resistance. Painting the fins also helps the UV thing. (But then you do not have beautiful clear fins....) Glass fiber filled PC is darn near indestructable (but not clear). It would make awesome fin stock of molded fins. Aerotech should shoot their fin molds with glass PC. It's a few bucks per pound versus the sub $1.00/lb impact styrene they use, but the fins would really survive.
Bulletproof "glass" is usually laminated PC sheets with PolyUrethane binder layers. Like a bullet proof vest, the layers/fibers are meant to fail, but each layer distributes more energy to cause it to fail. As the bullet destroys layers, there is less energy for destruction of the next layer. So hopefully it stops the round before it gets through the matrix.
Mid & HPR use? Absolutely! You should use thicker unreinforced (clear) PC sheet material than you would normally use for G10 fins. Use .093" PC when .063" G10 would work. Use .125" PC in place of .093" G10 and so on.
Good Luck