Some good advice offered above. I've also accrued some time on Twin Commanders, mostly 690/690A/690B's.
zlakarma - you mentioned getting a TC with the -10 conversions and in excellent condition. Some others mentioned the Grand Renaissance but you did not specifically do so. If your intended aircraft has not been through that programme, make very sure that it is in as good a condition as it seems to be before you buy. In my experience, certain things tend to get quite tired if the aircraft has not received the same level of love throughout its life. For example, pressurisation can be a pain so check your aircraft makes max. diff. (or at least whatever diff you need for your intended profile). I've flown many Commanders that simply don't. Even when the owners of others have said that the pressurisation is top notch, the pilots have sometimes told a different story.
Also the environmental sytem/air conditioning (particularly on the earlier models) can be a bit hit and miss, the refrigeration unit in the back can be expensive to replace if required and occasionally owners simply choose not to get it done.
I'd also echo SNS3Guppy's advice about high-speed descents. The book calls for it and I know many that do it, but I don't. A 180kt/1500fpm descent works out pretty good too and 180kts also happens to be the moderate turbulence penetration speed, so less panic if you suddenly enter some on a night-time or IMC descent.
Finally a fair few of the 690's still in circulation have been modified for aerial survey use. You may find odd holes in bulkheads and other mods if it has. If this applies to your potential acquisition, check they're all documented, particularly if you will need an export CofA.
I don't have any experience of the PC-12 to compare against. I'm not averse to single-engine types, however I do like an easy life and if I have an engine failure then I'm quite happy for that to be the only part of the emergency. On an S/E type, and engine failure also means a pressurisation failure/loss of generator power/loss of primary hydraulic pressure and so on. Making a crisis out of a drama....