I'm of the opinion that too much soot is a bad thing.
My parents had 1.9TD and 2.0HDi 90 306s almost back to back.
When I was driving them, I always felt like the the HDi was good, but there was something about the 1.9 that appealed to me, it seemed (perhaps it was all in my mind) that the HDi had electronics intervening in a cold and heartless manner, whereas the 1.9 had happy springs, clockwork and whirry gubbins controlling it. The HDi was much more cold and calculated to drive as a result.
Douglas