I just wanted to go into a little more about age as a limiting factor because I think it's being undersold.
As far as just looking at age by itself, there is no magic number where one year you are great and the next you are terrible. Age is a slow process that effects everyone differently. However, the overall decline is clear and it eventually comes for everyone. That means the longer you rely on a pitcher the more risky the proposition. Late 20s are a lower risk than early 30s are a lower risk than mid 30s etc. etc.
Since we can't determine when the decline will happen (or what kind of decline it will be) it makes a lot of sense to try to get as much production in lower risk time frames as possible. When looking at ZNN vs Scherzer in 2016 and beyond* the edge for ZNN is clear.
I've used the term "old 26" and "young 30" before. I do that because baseball reference considers a player age for a season to be his age on July 1st. You have to pick some date, that's the reality of it. While not perfect, their solution is a pretty good compromise between getting a date close to game #81 and getting a date that's easy to remember. However, you can see the problem that happens with having to pick a singular date. A player 29 turning 30 on June 30th would be considered the same age as a player 30 turning 31 on July 2nd, even though clearly they are more accurately thought of as being a year apart. To compensate for that I like to call players with in-season birthdays "old" and "young". If you have a birthday in-season before July 1st you are "young" and if your birthday happens after that date you are "old".
Really this is far more useful for prospects (being "old" vs " young" in the low minors can be the difference between elite and good) and only comes into play in the majors if you happen to be directly comparing a "young" player and an "old" player relatively close in age. Hey! That's what we are doing here! ZNN is a "young" guy (May 23rd birthday) and Max is an "old" guy (July 27th). And that means we are getting a big heaping handful of starts from a younger ZNN.
On April 1 2016 Max Scherzer will be 31 years and 247 days old (give or take a day on these things - leap years and all). Jordan Zimmerman will reach that same age sometime during February in 2018. Every start Jordan Zimmermann makes in 2016 and every start he makes in 2017 will be at a younger age than Max Scherzer will be to start next year.
To me that matters, that matters a lot. When you look at career pitching aging curves and see the declines that seem to be precipitous between 31 and 33, when you look at Hall of Fame careers and notice that they all pretty much peak in their late 20s/early 30s**, you get nervous. Sure Scherzer is a great pitcher right now, better than ZNN. But ask me if I want basically 30 and 31 year old ZNN or if I want basically 32 and 33 year old Max, I hesitate and land on ZNN. Ask me if I want basically 32 and 33 years old ZNN or basically 34 and 35 year old Max and I don't even hesitate.
I'm not saying it's wrong to see it another way. Maybe you care more about that Tommy John injury or think there is something special about Max that will allow him to be good longer*** I am saying that for me what I can feel sure about knowing is limited to the fact that in general as you age you get worse. That makes age paramount to me over any single injury and the age advantage for Zimmermann is big.
Now there's also another thing to consider. I did say in the beginning that I was "looking at age by itself". As Indy once said though "it's not the age, it's the mileage". What kind of mileage do these guys have and what does that say about them? Does that age plus something else lead us to look at a specific age in the future that may in fact be a point of no return? Tomorrow, my friends.
*I'm not asking here which is the better contract or who will give the Nats the most value over time. If you want to look at that you would put Max and ZNN a year closer in age since Max signed it last year. I'm looking solely at who I would want from 2016 forward. You can argue that "Max w/ ZNN for a year then Max alone after that" is better than "No Max at all and ZNN alone after" but that's a different question.
** What's up with Randy Johnson's career? Freak arm, am I right?
*** I tend to believe the opposite though - that ZNN's career that leans more toward pitch to contact lends itself more to a Mark Buerhle type 2nd act than MAx Scherzer's get them to swing and miss career.