Don't overfill them. For CO2, use a digital scale. For HPA, get it filled by a TRAINED and CERTIFIED person. There is a reason you have to have a dive license to fill SCUBA tanks.
Obviously, HPA is more dangerous (read my posts in the thread titled "nitro accidents") if it blows up. However, if you don't abuse the tank and respect the fact that it's basically a HUGE concussion bomb you'll be fine.
CO2 tends to blow up more often simply because it's sensitive to temperature. People leave them in the sun or a hot car and... BOOM!! The burst disk pops and there's CO2 everywhere, and possibly a large steel projectile travelling at a high rate of speed.
If, on the rare occasion, a full HPA tank blows up... well that's a different story. Go to the other thread I mention in this forum and read. Don't get scared, just learn to respect your gear... they're not toys.