Here's the European lineup with a general list of what's added to the higher model number. Remember, the US version of the same model number may be very different.
VSX-323 - 5 amp. 4 hdmi, 4k passthrough
VSX-423 - mcacc
VSX-528 - hdmi gui, internet / dlna / airplay, mhl, 6 hdmi
VSX-828 - 7 amp
VSX-923 - 8+2 hdmi, HD zone 2, 4k upscaling
VSX-1123 - zone 2 composite out. processing.
SC-1223 - 7 amp class D
SC-2023 - 9 channel preout - rs232, 12v
SC-LX57 - 9 amp