pre asph means it is the version of the lens before Leica started using aspherical lens elements. These elements help further correct lens aberrations, and generally increase sharpness while decreasing distortion, coma, chromatic aberration and so on. Some people prefer the look of the older lenses -- they like the way the bokeh looks, and prefer the softer look wide open. The newer lenses are sharper and better corrected, but some feel they look "harsh" in comparison to the older lenses. It is entirely a matter of taste.
The lens you link to is an excellent version, though you will likely have to pay a large price premium for the so called "king of bokeh". It won't finish at 400 something dollars, trust me! The best price/performance ratio in 35mm is either the Zeiss 35mm f/2 Biogon (by all accounts a fantastic lens), or one of the Voigtlander 35mm lenses (particularly the older f/1.7 model if you can find it...it is a better performer than their current 1.4). The Leica 35mm lenses are fantastic, but are extremely pricey. Only you can decide if they are worth the premium.