[DEERE317]

The biggest reason to re-power vs re-build is the fact that the key component of the tractor is 'new' - expected reliability is increased, the risk of "Now What" issues decreases, and they are better on fuel.
A lot of the members have > 1 tractor; so if one begins to act up, they have a back up plan to maintain normal routines. I wasn't in a position to own two tractors, I had to know the 318 was ready to go when it was needed. So for me, it was a question of upfront costs, and the resulting useful life of a tractor that was already 25 years old.
I decided to re-power. At this point, I am 10+ years in to my '15 year' useful life estimate, and happy w/ my decision - the engine has performed perfectly, w/ absolutely no issues (starting to get a little nervous about the now 36 yr old wires connected to TDCM though). It sounds like you use the your tractor for multiple tasks, so your 'useful life' timeline might be different.
Something else to consider - how much longer will other (non-engine) parts be available for these tractors, and at what cost ($$ and also 'time'). If you went the route of re-powering, would you have a plan B for the engine if your useful life estimate for the tractor came up short? If no, a re-build is probably the better option.

At least with Deere’s something that new probably has atleast 20-30 years of generally abundant part supply, the older 60-early 70’s machines I think are starting to hit the beginning of NLA land on some more idiosyncratic stuff but even then they’re still pretty dang good on availability compared to pretty much everyone else.

 

[DEERE317]

My hope is that parts will remain available and become more available. An abundant following of these tractors has remained and increased partly due to the wealth of information online. I noticed a similar thing happen with old Stihl chainsaws. Parts that were very hard to find, nla, are now being made (coils, sprocket covers with chainbrake ect).

Yeah have seen that for some odds and ends, my 185 really needed a new carb when I bought it but aftermarket was nonexistent and Deere/Kawasaki wanted a fortune. Over half a decade later their everywhere on EBay and Amazon.