i don't see these options as alternatives, but as directions. It is bad if one has multiple cores in a machine is not able to use it. The basic OpenACS infrastructure was built for multiple processing units. The point is simply: the micro-processor designers stopped a few years ago to increase frequencies and to add cores and SMT. These chips arrive now, Aolserver and OpenACS are ready to use them - NOW.
It would be certainly an interesting project to build something like OpenACS based on a P2P-network and DHTs (cassandra, bigtable, ... coral, cfs, oceanstore .... pier), but this will be a different project, since some basic assumption about the data storage are different.