I'll address your last point first.
I don't particularly want to live inside the perimeter. Smog isn't my thing.
There is no smog. To the contrary, Atlanta has the
densest tree canopy of any major American city. The air quality in the city is indistinguishable from that in the suburbs (I've lived in Atlanta and in the northern suburbs you mentioned). If you can't handle tons of pollen that'll be a big problem wherever you live in Georgia. If air quality only thing preventing you from living in the city, hooray problem solved!
At this point I'll go ahead and reveal my preference for the city. It's much more progressive in terms of bike infrastructure and walkability (and everything else).
Moving on to your suggested cities.
All of them are sprawling suburbs. Roswell has a nice dense downtown which is walkable and pleasant to bike in. It's got shops, a library, churches, etc. Downtown Alpharetta is similar, and they've even started adding bike lanes in the past few years. You didn't mention Marietta (NW of Atlanta), but it's also got a nice downtown.
Except those small (and expensive!) areas, the rest of the northern suburbs car-centric sprawl. The neighborhoods are great big cul-de-sacs attached by four lane 45 mph parkways (not bike friendly). There is one bike path that provides access to several office parks and subdivisions,
http://goo.gl/maps/KuwhV, but it's intended as a recreational trail and it passes lots of useful places without providing trailheads to get to them. If you're selective about where you work you could find a home that makes car-lite living possible, but it isn't a given and the area in general isn't conducive to it.
MARTA busses run along the aforementioned parkways in Roswell and Alpharetta and provide access to many of the office parks. Duluth and John's Creek don't have MARTA busses at all. Conyers is too far out to be served by MARTA, there's a regional bus system for commuters though. MARTA busses have bike racks on them. The route coverage outside the perimeter is pretty sparse, it's really meant to get people from the park-and-ride lots to the train station. Don't expect this to change anytime soon, service expansions into the suburbs are vigorously opposed by locals (something about black people using busses as getaway vehicles).