I agree with what you are saying, but however true that may be, all things being equal in terms of price, hours, warranty, etc., when given a choice, human nature as it is, people will purchase the tractor with the newest manufactured date. Not to mention, a tractor that has been sitting out in the rain and snow, freezing and thawing, in northern MI for the past 6 months cannot possibly be in the same condition as a tractor that has not endured those conditions.
I'm not suggesting most people aren't going to pick the machine with the more recent manufacturing date, I'm just saying that there is no such thing as a model year on a tractor, and no real need for one. For all practical purposes, a few months difference simply isn't worth worrying about at all. There have been times when un upgrade was made on an existing model, and they didn't change the designation, but it's fairly uncommon. Since we're only a couple of months into the year, I'd be willing to bet that some of the "2016" machines that farther dealer has could have actually been made at the end of 2015...it's not like the salesman was out there looking at manufacturing dates if they even know where to find it (many are shockingly ignorant about details like that).
In some ways you'd have more concerns with machines out west that get a lot more sun/UV damage, and higher temps that cook anything rubber or plastic. My previous pickup spent 13 years outside in Maryland, and the paint, dash, interior all looked really good...not perfect, but really good. I moved to NM and after two years the paint was cooked off the roof/hood and the dash and interior were starting to fade.
If six months exposure to the elements causes any measurable degradation in the condition of a farm tractor, something is very wrong.
There have been cases of dealers selling tractors that had sat on their lot for several years (without telling the buyer) and I think that's something that should be disclosed, but that clearly isn't the case you're looking at.
Further, if you buy from the dealer 1.5 hours away, he's probably not going to haul it there for free if you need service/warranty work, and if you try to get the local dealer to handle it, you may not get a great response from them.
It's the exact same machine, the local guy has a $200 better price....there's really no downside to buying from the local guy.