Around 1997 I rode a nearly new 900SS Supersport for a day, my first Ducati experience. Didn't like the buzzy handlebars at higher revs, even with gloves it was unbearable for long periods. Bike handled OK, but it felt as if it could use more power & have less vibration. Having been used to a Honda (VFR750) with sidestand switch I'd left it in gear when parked on a hill, then realised as I pressed the starter switch that it would still start!
If I hadn't let go quickly, disaster would have ensued...
I wonder how many people dropped their 900SS due to that thoughtful omission?
TBH I wasn't left with a lot of love for the 900SS, at the end of the day.
Assuming it's legal & OK to ride, take it for a spin for half a day before deciding. As you already have a nice RC51, it'll probably do everything better than the Ducati, so the chances of you actually wanting to ride the 900S by choice between the two bikes are likely to be low. Depends also on how much time you have for riding bikes...
...Your buddy should never have gotten rid of the VFR750.
Euro RC51 SP2: HRC WSB Ti Hi-Level, T1 Airbox & Snorkel, PCIII, Dymag CA5 Carbon Wheels, Ti64 Spindles (F, R & Swing Arm), Mori Link, Maxton GP7 Shock & Ohlins 832 Forks, Brembo GP Calipers & SBK narrow track (F), Braketech Disc (R), Brembo billet Underslung x 2 Brembo RCS, Tyga Triple, 7075 Stem, Probolt Titanium all over, Babyface rearsets, MR Complete Carbon Lineup, Custom Undertail & CBR 1000 LED, Watsen LED Indicators, Harris Brace, Giles clip-ons, Corbin seat, etc. 2013 BOTY.