From a technical standpoint, both are different throughout the smoker, but there are a lot of variables to take into account. For example, hot air rises, so the top rack would be hotter. BUT, if you have a nice sized brisket on the lower shelf, it will act as a bit of a heat baffle and take more "direct" heat This is less of a concern if your smoke chamber is 4 foot tall or so and you are using the top 2 racks.
As for smoke circulation, that kind of depends on the airflow in your smoker. Does it just vent straight up from the bottom, right out the top ? Or does it waft around a bit and eventually make it's way to the top? Also, is there anything sitting between the smoke pans and the first rack that meat is on (such as a water pan), which would diffuse the smoke to some degree and even out the smoke concentration?
With all that said, there are numerous other variables that will affect how a smoke session progresses, and how one smoke session might differ from another. Ambient air temp, wind conditions, humidity, etc. For example, say that you have 2 racks of ribs on the top rack of your smoker, one in the front and one in the back. Are the both the exact same weight/thickness (which will affect the cook time) ? Does the front and the back of the rack get the exact same amount of heat ? Is there a 5mph breeze blowing that will affect the thermal flow in your smoker and affect cook times?
All of these are reasons why you smoke your meat until it is done, versus following a specific time chart. Stacking more meat in will affect the cook times, but as shown above, so will lots of other things.
to mfalto:
Personally, I'd never smoke a brisket directly over ribs, or ribs directly over a brisket as I wouldn't want the flavors to meld. I want my ribs to taste like ribs and my brisket to taste like brisket. If I could arrange them to where one wouldn't drip onto the other, that would be different.