A low mileage bike is good if its been looked after. Anything under 25k miles is low in my book. If you can find a bike that has the add ons already paid for it will save thousands. Pipes, cosmetic, brake or suspension upgrades and ecu tuning can be substantial. Plastic tanks can be ruined by ethanol fuel blends and steel rusts so it depends. The M900Sie will have better suspension and brakes than the S2R800. The 900 is more old school and S2R pretty.
How big a rider are you? I found the S4 a roomy substantial bike and the air cooled versions very compact. Im 5'8" and 200 solid with short legs so I found the air cooled 900 a nice fit. A customer who is 6'6"-6'7" with limbs like a Mantis sold his 900 and got an 1100 Multistrada because at 50 he couldnt fold up anymore.
I prefered the old style grunt of the two valve motors for street riding.
Some Ducati specialists here will know details on when plastic tanks took over some model runs.
Good comment about preferences
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