People i know that have had th v8's have nothing but very expencive problems with them (mainly in the range rovers). My manager has got the disco(15 plate) with the v6 derv and rates it very highly, but then again he does not have to pay for any problems out of his own pocket.
The TD5 has been around for a good few yrs now and it seems to be standing the test of time quite well, so i would say your proberly best going for a tried and tested TD05.