I have both and agree with a lot of the points made already.
I only noticed just how much better the AR drum synth sounds are when I was traveling with the OT for a week thinking that was all I needed. Damn I really missed the AR solid kick sounds!
One really cool performance of the AR that isnt highlighted enough are the scene, performance and quick performance modes. Being able to take a simple 1 bar beat and present it in a myriad of ways is priceless and lends to the simplicity of it. I always recommend first timers to do this with a simple rhythm and use every single scene/perfomance button before making longer and complex loops.
With the OT I always have to have my notes close by to recall how to do various things where as the AR is pick up and go. Today I use the OT just for midi control and sample manipulations.