I don't have any reference but I do recall (and I believe this makes sense) that the pre-asph has sharper center, weaker corners than the asph. The pre-asph is breathtakingly sharp at 2.8 centerish, whereas the asph is pretty good.
Oh and there are some rendering differences too I believe. The asph is warmer and the pre-asph is cooler like most leica designs. If you have a pre-asph 35 cron, it would be a better fit.