If you know Nolan than you should probably know that there will be no sequel for Inception. But on the first place, why you need a sequel when the movie wasn't left as a cliffhanger?
I'm sure you're shattered in the end, whether the top will fall or go on spinning. Right? Well, Nolan particularly made it like that to see the interwebz going crazy to decipher it.
BTW, regarding your question, I believe the ending is in reality and the top would fall. But again, I'm not going into details as you can Google it yourself, and you'll come across plethora of web-pages where there are justification as well as open debate for ending being reality or dream..