I have both experience in wp multisite and in the wpmu commercial plugin. Multisite adds >400ms latency to the whole system, which you can overcome only by adding some solid server-level caching system. Wpmu doesn't slow down as much, and it can give you some nice added features like translation management. You can "translate" pretty much everything, including images and css. The url structure you need is available out of the box. I never tested alternative plugins.