were they bought 'off the shelf' or did you specify a shock by providing your body weight, riding style, pillion and luggage etc?
A bit different stories; the Nitron was ordered to specs - my weight, no pillion, road use. The specs for the Wilbers was similar, although it wasn't originally ordered by me.
If the Wilbers would be stiffer I'd see it as natural, since the Nitron was definately ordered for one-up road use (= no track) and I ain't that big.
Could the different rear frame/linkage make a huge difference between MkI and 9'er ?