i personally feel that as a single material printer the MK2S is superior to the MK3.
Now granted the MK2S does not have the new convenient features of the MK3.
However the MK2S has more stable firmware and better surface quality and less messing around,
basically the mk2s is a work horse.
less features means less things to go wrong as well.
the MK2S extruder design can also more easily handle softer materials like TPU. so i find the MK2s to have a larger range of material possibilities where the MK3 can be very finicky.
mechanically the MK2s is better and more stable. there is more to think about based on your needs. the MK3 removable bed does make removing prints much easier. the MK3 is also more octoprint ready compared to the MK2s if you know that you will be needing more remote management and have trouble with flat beds that are not removable then the MK3 will make more sense even with a more limited field of use. if surface quality is a must then the MK2s is much better then the MK3.
for me i have both machines i had a MK2S for over a year was was very happy with it. when the MK3 came out i was the first to get one. the Mk3 was a total disaster it literally took 4 months before prusa put out a firmware were the machine was half way usable. they were putting out revised parts constantly and nothing was working i rebuilt the machine from the ground up 2 times with no results. i had preordered 2 more MK3 machines when mine had arrived before i used it and found the major issues in the first week. believe me i was gung ho about the mk3.
Because of the problems i cancelled my MK3 order and changed it to two more MK2S machines and the MK3 machine went right back into a box for another 4 months as it was really un-usable.
It was not until this summer 2018 when the community had made enough progress in fixing some of prusa's issues that it made sense to start using the machine again.
even then it has been constant tinkering with the MK3 with little improvement. however since the machine is basically experimental and i had other printers working on output i decided to try out a MMU2(multi-material) upgrade for the Mk3. i have been using that with some success.
with remote print management i had big issues with the MK2S. under the same conditions the MK3 worked ok.
so food for some thought. it will depend on your needs or future needs.
print quality = Mk2s
less break downs= Mk2s
range of materials = mk2s
speed = mk3
remote management = mk3
easy in removing prints = mk3
filament management = mk3 (spotty)
better energy management = mk3
faster start ups = mk3
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