Not comparing the technical merits of the two aircraft (not worth it and has been done to death here I am sure) I am curious about how this will play out in the market and development side of things over the next 10-15 years. Now that both programs are on pretty firm footing it seems a good time to look at what happened and what is likely to happen over the life of the programs.
I think Airbus was very smart in doing the NEO because I fully believe Boeing would have been content to just stick with the older aircraft and work on the technology to do something totally new. I also think that the fairly minimal changes other than the engines on the part of both manufacturers reflects that neither really expects to sell these things for a terribly long time. You keep hearing warnings against "mission creep" and a general tone of wanting to do enough but not too much that would make the programs have much technical risk.
In my view both programs were really about preserving cash flow for both companies. Airbus initially could not afford to have customers sitting around waiting for new aircraft, particularly given that its engineering resources would be fairly tied up in the A350 while Boeing would presumably be freeing those resources from the 787 program and able to apply lessons learned to a new plane in the narrowbody market. Boeing could not afford to cede that market to Airbus for the time it would take to develop something wholly new so they jumped in as well.
As almost an afterthought to this I think both companies discovered that doing this would effectively prevent any other manufacturers from jumping into the A320/737 sized markets. Assuming there was a technology gap (ie the tech for a new narrow body was not going to be ready to start development until 2015-2020 anyway) then someone might have jumped in with a new efficient narrowbody in the interim. The NEO and MAX pretty much precluded that and will likely ensure that really only Boeing and Airbus are competing for the next jet in that market.
As I look at the situation now I think you have to expect one manufacturer or the other will look to a new plane in this market in the 2018-2020 time frame assuming the technology is mature or close to it. Both companies will need to retain their engineering skills so there has to be some project to work on for them in the commercial field. With the 787 and 777X out there Boeing won't be looking to do anything in that market I would not think. Both will be too new to mess with. Airbus will be in much the same boat I would think. The A350 will be brand new and replacing the A380 would not seem to be economically viable at this point. So unless you want to start losing lots of valuable skills it seems to me that both companies will be positioned to do new models in the short-haul market.
It strikes me that the NEO and the MAX seem likely to have fairly short production lives compared to the designs that came before them. It seems that both companies are accepting of this in that both seem focused on minimizing cost going into the designs. Both companies seem positioned to throw engineering resources at the problem in roughly the same time frame (I think this was the biggest benefit Airbus got from the NEO frankly). Should be interesting to watch at the very least.