Which truck to choose?

Check the paccar,engine, see if it a first or second generation engine. Check the build dates.
If it is second generation. It may be a better option than the same year cummins. Cummins had a lot of emissions and wear problems from 2010 to 2015. They are fixable, but not cheap, and it may actually be better to do a full overhaul of the engine. Then you can do the upgrades and fixes to get,a solid motor.

As to the sheetmetal, you choice but the 587 will run a tad heavy over the 660. For the most part whats under the sheetmetal is almost exactly the same.

 

Halburslegben Trucking, where I keep my truck, runs 50 late model 680s & whatever the Pete cousin is.

They do refer, always heavy, the have all MX13s, and love them.

I brought this up specifically to the terminal manager there to get some real world feedback.
The owner was there as well and he had nothing but good to say about them.
They have about 200 trucks total.

They said 0 mechanical issues or after treatment issues.

They said the bug is the tire damage system. You hit any bump, truck thinks crippled a tire, and it's interlocked with the transmission.
So they had that system removed, but your ECM has to be updated.