search around the forum you'll see various people (other than me ) who have experienced both, say when getting car serviced and being lent the other, or actually choosing to change between the 2 (say, to get facelift interior). The conclusion are fairly consistent...

for me, it started with trying out the 2.7TDV6 in 2009 vs the TDV8, then trying the 3.0TDV6 in 2011 when changing to a 10-plate TDV8, finally have a little go in the SDV6 earlier this year.

most obvious is the torque delivery between 30-70mph. The TDV8 is just throwing more at you, for longer (check the curves). Admittedly the stark contrast between the 2.7 and 3.6, and the 3.0 (TD) and 3.6 has decreased for the current 3.0 (SD) and 3.6 - but the difference is still clear.

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2010 TDV8 HSE - ooo laa laaaaaaaaa
2007 TDV8 HSE - superb car and very reliable
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2012 - dearly departed, sadly missed.. to return in late 2013 with a new TDV8!