The RXV620 is basically just a DSPAX620 with a radio tuner!
Some super pursists might say the presence of the tuner may affect the performance - personally I think that's a load of old twaddle these days, but then the audio world is also full of people with superhuman hearing, so you'd have to make your own mind up on that!
i bought the DSPAX620 yesterday from RS and it's totally fab! i spent most of the night looking like the maxell man!! i cant comment on the new version of the amp except to say that its twice the price (but is it twice the the performance?) if you need a tuner then go for the RX-V620RDS, but i can assure you its a great peice of kit for £200.
I recently owned a 620 and yesterday heard a 630 and as far as DD and DTS are concerned in a 5.1 setup they are almost identical sounding. For 6.1 or if you watch a lot of telly (Sky) then the 630 is the one. 630 is also slightly "sharper/cleaner" sounding in stereo. DPII is a BIG step up in the 630 over the DPI in the 620. A tuner choice comes down to the individual.