I currently own two DS109, one DS209 and one DS211J.
IMO, the regular models (i.e. DS212) is sufficient for most home or small office use. I wouldn't recommend the economic "J" models for anyone who might be running multiple applications or really doing anything more than streaming to one user at a time. It's slower to wake up from sleep and is the only one of my synologies that have had any issues whatsoever (the external USB ports lose connection every once in a while). That said, if you wait an extra five seconds for it to wake up from sleep, the J models can stream an HD movie as efficiently as the regular models, though perhaps not more than one at a time (I've never tried).
Between the regular models and the "plus" models, the difference is negligable unless you really intend to max out all of your synology's capabilities. Let me put it this way. My DS209, which has a slower processer than the DS212, has been able to stream three different HD movies simultaneously, while I running a sync backup at the same time, without any noticeable lag whatsoever. I also run a photo server off that unit and while it is slow to process the images when you initially load them up, this is a function of the processer, not the memory, so you would get no boost from upgrading to the "plus" model. If your uses are anything like mine, I wouldn't bother with the plus model. Get the DS212.
Lastly, what you heard about the "slim" is true. A friend of mine bought one based on the price without reading the reviews, and quickly discovered why the price was so low. This model was a failure and is priced inexpensively as a result. I don't envision there will be a DS412-slim, or at least, they will have to make substantial modifications if they plan on continuing that product line. Stay away. I'm planning on upgrading to a four bay unit soon and will likely go for the DS411 as the DS412 is not out yet.
Hope this helps.