So I've looked at the previous threads on interior paint and I am trying to figure out what level sheen would be the best solution for walls (living room, bedroom, kitchen, bathroom).

I believe that flat paint for walls is the cheapest option because it is easy to touch up will in the end produce less labor costs when paying painters etc.  I will also look nicer when freshly painted and help hide imperfections in the walls. If I plan to use a flat paint I would still have to use a different paint in the bathrooms with an eg-shell shine.  In my opinion there is a reason why 90% of apartment complexes use flat paint.  However, most of us don't have maintenance crews standing around waiting for something to paint.  This paint is likely ideal when units turn-over quickly.

Eg-shell seems to the advantage when it comes to cleaning the walls and using the same paint throughout the whole property.  Eg-shell would likely cut down on material costs because you don't have to worry about putting it in bathrooms and kitchens.  However, the look would be slightly less appealing and whole walls would have to be painted when "touching up".  This also means that walls will be more durable and they can look good for a longer period of time.

So with running a small operation of a few higher-end rentals and trying to increase the longevity of the tenants, what would be the best option?

Please feel free to share your methods and/or colors that you use!