I'd buy used. I don't think the Summicron is worth whatever they charge for it new. Heck the ASPH isn't worth what they charge new for it either. Look around, you should be able to get the APSH for $2-2.5k. I imported mine (new) last year for less than $3k. At the beginning of this year, you could get them new from the UK for less than $2k.
The ASPH is a bit bigger, faster, and more resistant to flare. Otherwise, it's a stop faster and about as sharp. It might be a hair softer than the Summicron, but it is probably more even across the frame than the Summicron, especially stopped down.
I don't know how they compare on things like coma, astigmatism, etc.