I think the marketting info may be a bit out of whack ... certainly (at time of writing) there are no models on the 3/400 that are not available on the 500 so don't worry about that.
Really it depends on how ambitious you are for your setup. The simpler you like it, the more likely the HD3/400 will fulfil your needs. What you pay for with the HD500 is flexibility. Experienced users of MultiFX value this very highly and are willing to pay the premium. A number of people on this forum have started off with an HD3/400 and traded up once they hit the limitations.
If you want flexibility then your really do need the HD500. The ability to place any effect anywhere in the 'chain' is a really big deal for some people (e.g. me) and that is just not possible with the 3/400 where certain effects are tied to certain positions in the chain and cannot be duplicated.
On the down side, the HD500 is definitely more of a challenge to get setup and working so if you don't have the patience for that then you need to consider the tradeoff. There is loads of great info and resources on this forum to get you up and running but few people get what they were looking for on day #1. Typically, people get up and running faster with an HD3/400 (from what I see posted) and many people are quite happy to stick with it.