If I've read this correctly, you're comparing an A/V Receiver like the Onkyo 906, to an A/V processor plus multi-channel power amp like the Arcam AV9 & P7. Correct?
All in one box has got simplicity of connections on its side (Yes, even with the amount of connections they have!), but primarily cost.
An A/V processor plus power amp is gonna set you back a few thousand at least, as only the high-end companies seem to do them. If music is important to you, then you could always use an A/V processor in conjunction with a brace of 2 channel power amps, but again we're talking a fair chunk of money here.
Budget is gonna be the deciding factor, also, what speakers are you planning to use with it? If you're gonna go down the style speaker path, the separate processor/power amp is gonna be overkill.
I don't doubt that A/V processor/power amp set ups are superior, but I also believe you can buy truly astounding A/V receivers that do the job almost as well.
If I've read the question wrong. Please disregard the above.
;-)