Nothing....IF you want the easy flexibility of Atlas and the ability to easily slide the rails through the tie strips to stagger your rail joints. If those two things are not a concern, the shape of Micro Engineering's rail is very similar to that of Atlas. ME is somewhat harder to curve. It has more "spikes" than Atlas, and it is near impossible to slide the rails to stagger your rail joints. The two brands are compatible.
I have NO financial stake in either, and I'm not advocating one over the other, just trying to accurately describe their characteristics. Old Pullman made a flex, but I've heard they are no longer in operation....not sure how accurate that statement is, nor am I familiar with the characteristics of OP flex. I see the different brands advertised in private sales from time to time.....you could try that route.
If you decide to buy ME, you might want to check with Ed Duddy (House of Duddy) to see what he might have. Ed is the person that put up the money to get ME code 148 flex produced and a heck of a nice old guy. ME flex used to have "House of Duddy" stamped on the back of the tie strip.....not sure if that is still so.
Simon