Without seeing the balls, knowing the layout, seeing you bowl, knowing your stats, surfaces of the ball,  we're pissing in the wind for lack of a better terminology. The only constant between the 2 balls, as I was told by a reputable Brunswick staffer, is the core. They share the same cover and what separates them is the core.
As someone who's had both balls, I can agree that the Reax offers more "pop" on the backend than the Nightmare. The Nightmare got into a roll earlier than the Reax and my Reax is drilled slightly more aggressive.

By your stats, I meant speed, rev rate, axis tilt and rotation. Where on the lanes you are successful with the balls? Are the layouts the exact same, as we are dealing with Asymmetrical balls, slight changes can cause different reactions (with the same surface and position of x-hole). Do they both have a weight hole?
I was told they have the same cover. Not saying you or anyone else is wrong, but I trust my source and anyone is free to tell me otherwise with some sort of proof. And if you want the Reax to be stronger then change the surface. It loves surface changes and has worked well for me with a straight 500 to 4000. If you dont want to keep making surface changes, then get another one and set it to the surface that works well on the heavy sport patterns.

They are not the same cover. Similar resin base different additives.
I think I should be considered a vaild source 

Both have a balance hole...my rev rate is approx 350-400...I play the same part of the lane if possible...up the 8 board, slight swing to 4 or 5...If I have to play the middle, the Nightmare is stronger, If I have to play up 5...the Nightmare is stronger. Nightmare at 4000, Reax at 1000.

Both have a balance hole...my rev rate is approx 350-400...I play the same part of the lane if possible...up the 8 board, slight swing to 4 or 5...If I have to play the middle, the Nightmare is stronger, If I have to play up 5...the Nightmare is stronger. Nightmare at 4000, Reax at 1000.

ok, both have a balance hole... but what position. 2 different core, 2 different covers(thanks phil ;-) ), 2 different surfaces. that more than likely is the difference.  1000 grit on the Reax makes the ball puke while 4000 makes the Nightmare come to life. 

If 1000 on the Reax makes it "puke"....why is it a great ball on a lighter condition? On the heavier volume it dies. I'll take it to 4000 and see what happens.

Sheer luck.
House oil patterns tend to hide the difference between balls. You could be getting just the right amount of midlane burn to control the Reax.

Please re-read Phil (newguy)'s comments; he is the designer of the ball!!
Are you on the true "heavy" oil, that is almost required by the Reax, or are you on oil just heavy enough to to allow the finer surface of the Nightmare to perform??