The 4.0 twin-turbo V8 CDi engine suits the W164 much better than the 6.2 V8 in my opinion; the ML63 I took on an extended test drive left me feeling underwhelmed, it was a shame Mercedes dropped that diesel engine.
I was watching the ML63 prices for a while and some have been hanging around for a very very long time, but on paper they should be good buys really for certain folk; north of 500bhp N/A V8, 4x4, high driving position etc