I think you're right, there's not much of a difference between these rims. They both build up nicely, both are old, proven designs, and both are very durable.
The MA3's are a bit heavier, and I think that's because they have slightly thicker sidewalls. If you're going to have brakes on your bike, that could be a good thing, as the sidewalls may last longer. If you're building a brakeless fixed gear, then you might want the lighter Open Pros. The difference is pretty small, though.
I've got a pair of 36 hole MA3's laced to Suzue Promax hubs with 14/15 db spokes, and the wheels are absolutely fantastic for city riding.