Well I took the plunge and got some of the new opnext 637nm 170mw diodes. I can't really see much difference in the wavelenght as expected. I find the diodes is about 20-30% brighter (hard to say by eye)then the 642nm diodes. They could be a dead even match. The difference is not much.
The color mixes well. The size of the beam is the exact same as the 642nm diodes. I'm running the diodes at 300ma. I have no power meter or spectral analysis to offer just my eyes. IF you are starting out go ahead and buy these. You won't be sorry. If you already have 642nm diodes don't upgrade. Wait for your diodes to pop and then replace them with these.
Truely if I could get 1.5w 660nm in a 3mm round beam, I'd pay dearly for it. 800mw of 637nm vs 600mw of 642nm I would not trade up.