Thanks OP for putting this analysis up. My wife and I were after RDX earlier this month but ended up buying the NX instead. We were in-route to test-drive the RDX for my wife and passed the Lexus dealer on the way to Acura. We had some extra time so decided to stop at Lexus dealer to check that NX a bit further. We test-drove first the NX, then RDX within 2 hour period and ended up buying the NX. Here is what won us over:
1) Exterior Styling. The NX has a fresh new styling both inside an out. The exterior design is very modern looking. Some may be put off by bold spindle grille but surely everyone on a road pays attention to NX. For RDX - not so much. As someone posted, RDX design is pretty bland and the vehicle looks thoroughly outdated.
2) Interior. The NX's interior is very high quality and has that enveloping feel. The controls are well placed and multi-media system is great (but as every v1.0 has it's own quirks). Both my wife and I found touchpad interface very well executed and convinient to use. The interior space is plentiful with front seat long travel back. The seats are also very comfortable without being hard or restrictive. In RDX the interior is pretty average and does not look nor feel like the lux SUV it is. The multimedia system rotary interface feels antiquated and it's location on the front panel is not very convinient - another easy win for the NX.
3) Road feel - actually this is where both cars feel pretty equal. Both has very composed ride and in acceleration Acura actually wins. Couple of times during our test-drive the RDX felt overly stiff. I personally wouldn't mind some stiffnes for SUV (an oxymoron, right?) but my wife liked Lexus balanced feel much better. The acceleration is where the RDX really excels, but again, the NX, is not that much far behind. From the moving start NX feels much more nimble thanks to its turbo.
4) Fuel consumption. On paper NX is better - 22/28 mpg vs. 19/27 mpg for RDX. From the forums I gather people easily make 24mpg on the NX. RDX real life MPG is around 21-22 so here again the NX wins.
5) Number of options offered. The 2015 RDX I was looking for did not have BSM or Parking Assist options at all (these would be availabe at 2016 RDX) while the NX had an array of options. Yes they would drive the price up, but it's good to at least have an option.
All in all RDX is a great car in desperate need of updating and redesign while NX is well-rounded, very nicely put together and is simply better all around.