In my limited experience Iíve worked (or interned) for all three.
Iíve Interned in Rural Areas. (underapreciated)
My first job out of college was a temp position with a county of over a million people. (un-rewarding, very politcal boss, but fun place to be!)
And now I am in an outer ring suburb. (semi-rewarding but boring)
My ultimate goal is to get back to the city but the position I had there was mainly research for things that will never be adopted. We performed more like a regional council then a planning department. In other words, the work was not as involved and rewarding as in the smaller areas I have worked. But I was happier being in a large area with more to do. More respect was also associated with the position in the large area, when you told people you were a urban planner they knew what it was.
What I am asking is what is the preferred place to work(in your opinion) Large Area, Suburbs, Rural Area