Thanks a lot all of you for your insightful advice.
The FLI would definitely be my first choice if it wasn't for the price. If I went for the FLI then I would still need a separate filter wheel, a more expensive 50mm filter set and an off-axis guider (or guide scope etc.)
The QSI is pretty much unbeatable with a total price of US$4140 all up, including installed Astrodon filters.
Regarding the cheaper alternatives, QHY etc, I just think the QSI offers absolutely the most value for money, being an all-in-one solution of high quality materials throughout.
As for narrowband filters and not having space for them in the in-built filterwheel: It's a good point. Personally I have never tried NB, but if I go down that path then I suppose I could have separate filter carrousels with appropriate filter sets. For the time being my main interest lies in capturing RGB images so I'll worry about NB later.
Regarding vignetting, I expect it won't be too bad since I'll also likely be using a ParaCorr which would turn my f/5 into f/5.75. The QSI is advertised as being free of vignetting down to f/2.8(!) - Don't know if it's true in practice though?
As for the cooling, I'll probably wish I had the FLI on those few nights that are particularly warm and muggy here in NZ, but most of the year I should be OK. If I had to, then I woudn't mind trying out the watercooling. I have a permanent setup in my obs so could probably find a relatively safe place for buckets and hoses etc, but of course I'd rather be without.
All considered I think that the QSI will be a good performer as an initial step up from the ToUCam. That in itself will simply be a massive boost to my imaging capabilities. I'm currently imaging with a 640x480 webcam chip with approx 1/3 the sensitivity of a 'real' CCD camera...
There is just no comparison.
Again, thanks for all your comments. It's great to have such knowledgeable people at hand when making these decisions.
I'll probably make my purchase sometime around April/May next year, so I still have some time to think it over.