I'd agree with you that the 90mm Summicron is a better lens than any of the f/2.8 options (unless size/weight is the over-riding factor). The older f/4 Elmar has it's own charms, and would be my choice amongst slower Leica lenses at that focal length. As Brian wisely says though, don't ignore Kobayashi san. The Voigtlander 90mm/3.5 Apo-Lanthar is said to be a cracking lens for the money.
I'm also in full agreement with Brian on the merits of Leica's current lenses; on the merit side they are amazingly sharp, but to my mind are somewhat clinical compared to the older lenses. Given the modish perception of today, that technical quality assumes a far greater dominance over photographic communication in a picture than is good for it, that's fine. Personally I'll opt for the older glass, even with their abberations, 99% of the time. They have character in bucketloads.
-------------------- Tim BSRIPN
You see something happening and you bang away at it. Either you get what you saw or you get something else--and whichever is better you print. - Garry Winogrand