New Technics turntables

Different strokes for different folks.  You've got good reasons to do what you want to do, Abery, but if your requirements were not what they are, the point is that the 1200GR is almost certainly the "better" turntable, given that the differences in favor of the 1200GR may not be meaningful or useful for your needs. In other words, go for it.

best-groove, If you can re-interpret the following sentence fragment, I might have a better idea of what you're trying to say: "...has alternating the cartridges he possesses on both and not he noticed better differences passing from the SP10 vs 1200 GR while with different arms."  

What was your friend's thinking as regards the stethoscope experiment? (I'm a doctor, too.)  Listening to music using a pair of familiar speakers driven by a known amplifier is the test, assuming neither turntable is grossly malfunctioning.  And yes, one must do the comparison with the same tonearm and cartridge on both turntables, for the results to have meaning. I realize that may be inconvenient if not impossible, because the GR tonearm is fixed.

Finally, I am not at all concerned about the future repairability of any of my already ancient DD turntables.  They are all beautifully made out of materials that will never degrade.  The only moving part that wears is the bearing, and bearings can always be renewed.  The motor and associated electronics are made of parts that can be sourced without recourse to the company that originally built the turntables.  What I would worry about is the continued availability of smart EEs who understand how these things work and are willing to fix them.

Chakster, Why is older always better?  There are many reasons to posit that the best of the G series might be superior to any SP10 Mk2.  And I say that as a past owner of two SP10 Mk2s.  I am thinking about the coreless motor in the G series, as opposed to the iron core motor of an SP10 Mk2.  I think we both agree, or at least you have said it in the past, that a coreless motor is to be desired in DD.  I admit, older is very often better, but not always.

To the OP, if you are an audiophile, get a G series Technics, not the SL1500c, regardless of cost.