In practice, what are the advantages and disadvantages upgrading from i3MK3S+ to MK3.9 or MK4?  

RE: In practice, what are the advantages and disadvantages upgrading from i3MK3S+ to MK3.9 or MK4?

Disadvantages of the kit route:

• The cost of the upgrade isn't that much less than buying a new MK4 kit.
• You can sell the MK3S+ for more than the difference in the cost between  the upgrades and a new MK4 kit.
• You can keep the MK3S+ and buy a new MK4 kit and have two printers.
• You can find parts that need to be replaced during the upgrade such as a warped heat bed or X / Y bearings and rods.
• Shorter wait to get the MK4 kit.

Advantages of the upgrade:

• Less hassle since you don't have to sell the MK3S+ or find a place to park it.
• Less total money outlay.
• Avoids the temptation to upgrade the MK3S+ to make it as good as the MK3 kit that you bought.
• Fills up the parts drawer with MK3S+ parts waiting for a project to use them on.

RE:

Due to space limitation, I can only have one printer in my room. How about print speed and quality?

RE: In practice, what are the advantages and disadvantages upgrading from i3MK3S+ to MK3.9 or MK4?

• The prints off my MK4 look better than my MK3S+.  This is due to the new stepper motors eliminating the vertical artifacts that the MK3 family has.
• The MK4 prints a little faster than the MK3S+:  A 3d Benchy on my MK3S+ takes 1 hour 21 minutes.  The MK4 takes 1 hour and 9 minutes to print it.
• The MK4 seems to print tighter tolerances than my MK3.  This is based off only a couple of prints so it might not be true.
• The near perfect first layer is the improvement that I like best about the MK4.   No more doing the live z adjustments and having to change the live z setting if you changed between different types of steel sheets.

If you want the best looking prints you would need the  MK3S+ to MK4 kit.    

RE:

Thank you. The print speed does not seem to be shortened a lot. I am doing fine with manual first layer calibration. How about ease of changing X and Y belt tensions compared with the i3MK3S+? As for changing the nozzle, is it as simple as the Revo 6?

RE: In practice, what are the advantages and disadvantages upgrading from i3MK3S+ to MK3.9 or MK4?

No difference in the difficulty of  adjusting the belt tension.   There is an app for the MK4 that determines the correct tension by the sound the belt make when you strum it with your fingers.   I found the belt tension meter works just as well.   https://www.printables.com/model/46639-tension-meter-for-the-gt2-belts-of-i3-mk3s-or-prus

Not much difference with the nozzle change.   You unplug the hotend wires from the Love board and loosen a couple of thumbscrews to remove the hotend.

No need to have the nozzle and hotend hot to do the change.   You do have to make sure to get the wires routed right when putting the hotend back on the printer.  Once you do it a time or two, it isn't hard or time consuming to do.   It makes it really nice if you have to change the thermistor or the hotend heater.

My MK3 has a little over 11,000 hours of printing time on it.   So that made it easier for me to justify buying a new MK4.   It all comes down to what you can afford and what you want to do. 

RE: In practice, what are the advantages and disadvantages upgrading from i3MK3S+ to MK3.9 or MK4?

The MK4 is already much faster than an MK3.

The final input shaper version should be released this week. 

If I had a MK3 I would just install Klipper. A Raspberry is cheap again and the rest is software. With that the MK3 prints as fast and nice as a MK4. Only the fan is stronger on the MK4. You may have to upgrade. 

Or sell MK3 and order a kit.

If you are comfortable and want to stay with the Prusa software, the 3.5 upgrade makes the most sense to me.

RE: In practice, what are the advantages and disadvantages upgrading from i3MK3S+ to MK3.9 or MK4?

I have an MK3s+ and I thought the first layer was nice, not needed.  Speed I thought was a wash.  Then I got my MK4.  See my notes on the overall improvements, it is a major change.  I plan on upgrading the MK3s+ to MK4 as soon as I can get the upgrade.  The upgraded MK4 will go to a family member.

In my opinion the MK3s+ was a great machine, but the MK4 allows more people to use a 3D printer.   

I have tested the Input Shaper, firmware 5.0 release candidate 1.  It is faster, but limited so far to 0.4mm nozzles.  It will be further development/data collecting to include more nozzle sizes.