I only carry folders when I'm in the city or some place where they are clearly forbidden. Fixed blades are, in my opinion, superior tools. They don't have complicated mechanisms to be strong, they are easier to clean, etc. Their only disadvantage, in my opinion is that they are illegal in some places or not socially acceptable (I can teach a class at uni with a huge Talwar XL in my pocket while a small fixed blade on my belt would not be acceptable).
I've found 4 inch blades to be about ideal for my needs, sometimes I use something a bit smaller and sometimes a bit larger (say, between 3" and 5"). I don't really use "neck knives" because most of them don't have a good enough handle. I'll occasionally carry an Izula or BK-11, but I much rather have something with a full size handle.
I still think folders are cool, and own a lot of them (and will of course keep buying more). They are easier to carry in the city, pocket fixed blades are nice but usually tiny (with handles that aren't suited for hard or lengthy use) with a folder you can still get a nice blade length and handle size in a very compact package.