I think I've already covered all the areas I had in mind about these two operating systems. And as the thread has lasted for too long, I'm concluding it now. Here is a summary of all items described:
After all these notes I still can't decide which operating system I'd prefer based on quality, features and cost. Nowadays I'm quite happy with Kubuntu (installed it to see how it works after breaking Ubuntu and it seems good so far) and I'll possibly stick to it for some more months.

This will last until I feel the need to buy a Mac again (or simply renew my desktop), at which point I might buy one with Mac OS X or wait until the desire passes away ;-)

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