Hi Gainful,
I agree if the power got put down at lower speeds then you'd get more wheelspin. I don't think that's really petrol vs diesel though, just more power vs less power, whether from petrol or diesel or gear ratios.
You can do about 30 in 1st in the petrol Fr and I think someone said it was about the same in the diesel, so the different revs and torque should "cancel out".
for example, at 10 mph they'd both be producing 1/3 of their total power, at 30 mph 100% of their power, so the difference in acceleration is just that the petrol is a bit more powerful at each speed in 1st gear, so if there is sufficient grip to put the power down it can accelerate the car faster.
Just a thought anyway