For starters, due to my own personal preference, I've been looking at S*R models for the adjustable suspension and the single-sided-swingarm models.
Also trying to compare apples to apples as much as possible, I've done some reading about the S2R1000 versus the S4R. It really seems like the S2R1000 is a more suitable street and day-ride bike than the S4R, it seems like the 4 needs to spend all its time wound up high to take advantage of the power. I've also read that the 2 has a more usable torque band as a result of not being so high strung. The bike I end up with will be used mostly for road and spirited twisty roads, with occasional track days, so something with a usable power band is important.
It's a tricky point to understand without riding them, and it is not all about "power". The S4s make similar power at similar revs to S2s of the same size, but on the S4s the power keeps on increasing after 7500rpm all the way to 9000 or 9500, whereas on the S2s it tails off pretty quickly. So one question is whether you will run it past 7500 often enough to justify the extra expense of an S4.
There is a secondary aspect to this, however, which is more connected with feel than power. The S2s won't have much more power down low (i.e. on a dyno) but IMO they feel more responsive down there, because the intake system is a bit smaller diameter. I won't go into the fluid mechanics, but you will
feel this as more grunt. The upside with the S4 is a sense of having more headroom: you feel like you are using less of the engine's potential when just cruising along, and especially when cruising along at 85mph or more.
Add that up and the S2s tend to feel better in the city and at least as good on tight twisty roads, while those who like the S4s enjoy the way they feel on the open road and when the curves open up a bit.
So, "spirited twisty roads"? Depends a bit on just how twisty. Occasional track days? Again, will it be slow, twisty tracks or fast, open tracks - and how fast will you be?
Another question for the experts is, in regard to any used Monster, assuming proper maintenance and having been well cared for, is a bike with 10-15k miles on it a safe bet mechanically? It sounds like it is, hearing what you guys have said about bikes that finally after 40k miles need a fresh engine. That's another reason that I've read a bit more about the S2R1000, the fact that its engine is (based on the reviews) based on the very well-proven, robust engine layout that's been in use forever.
Are there any folks here who can vouch for the S*R models? Anyone who wouldn't vouch for them?
IMO, 10-15K miles will be nicely run-in. I'm at 53,000km on my S4Rs (34K miles) and I know a senior member of this site has much more on his. I think the essential mechanicals on S2s and S4s are very durable. The S4s cost more for valve jobs, but the main issue with mine has been with the radiator: the original and its replacement have sprung leaks in the same place from poor design mixed with vibration. From talking to repairers and seeing the odd thread, this is a widespread issue with Duc radiators as the mileage adds up. If you can find a good repairer, they are repairable cost-effectively, but it's irritating and Ducati wants an arm and a leg for new spares. Obviously the S2s don't have them. The other annoyance has been Ohlins fork seals. Think I'm on my fourth set in six years.
So ... what sort of performance do you want, and how much will you feel comfortable paying for it in terms of dollars and care? Possibly your key quesitons.