I am a big fan of the CS-400. I also have it's replacement in the Echo lineup, the 4010. I haven't run that one enough to form a strong opinion. It's slightly bulkier than the 400 with a few plastic pieces that seem flimsy. I like the filter and top cover better on the 400. I also like the 400's controls better. The 4010 has spring AV whereas the 400 just has rubber. The 4010 also has a strato engine so maybe it's better on fuel.
For me, 40cc is a niche saw. It's the perfect size to throw in the truck for hunting or construction projects. It makes a nice companion to bigger saws when cutting firewood. With a muffler mod and a little tuning the CS-400 is a sweetheart. I don't run mine the way a pro would, in fact I baby it. In my limited experience, Echo makes way better consumer level products than Stihl or Husky. Compared to the current Poulans and Homelites, etc it's not even close.
All that being said, it sounds like what you want is the Husky. Buy what you really want the first time and don't look back. Personally, I would never use such a tool to it's full potential but I would still appreciate the quality.
As for Echo's new naming convention, it works like this: "XXYZ". XX is the displacement, Y is the model, Z is the model revision. It looks like they are dropping a lot of older models lately and the new ones use this four digit naming scheme. The "X" Series are their pro level tools, though I am not sure they can really hang with the pro level Stihl and Husky products, at least where saws are concerned.