The focusing speed no difference. The IS lens is newer than the non-IS and also boasts weather sealing but no pt if your body ain't weather-sealed though. The IS helps a lot if you tend to use handheld in low light situations. IQ is almost the same but some ppl say the non-IS is slightly sharper than the IS.
Indeed, but its the only way to full frame for the FF enthusiasts out there. And the IQ is quite a bit better than 20D/30D.4.5k is rather ex for body only.