The diligent users would only choose reputable software vendors and always verify their software (md5, certs, etc).
Indeed, I wish more developers would provide hashes & certs.
Unix/Linux wont be the panacea for solving all your security issues unless you have sound technical knowledge to deal with them. If you don't have the time or inclination to learn it all then look at OSX first then Windows.
Good summary.
You have to remember where Linux/Unix came from, primarily intended for use on servers and all taken care of by one large community of coders. You have to remember that everything is a separate software package coded by different groups of individuals, all just handily pre-compiled and distributed for you in one friendly package. The user interface is just another package, perhaps with a few custom "themes" to match the Linux distribution colours.
Whilst OS X does have open source behind it, you have to remember a lot of that code came from Apple are ploughing R&D money into developing and maintaining it with whole armies of coders.
Hence the more polished look and feel of OSX, as well as the additional behind the scenes technical enhancements. Apple have integrated it all very well indeed.
Of course, OS X provides you easy access to the command line too. So you get the best of both worlds essentially, which you know will never be there because of the way Linux distros work.
I've tried the latest Ubuntu desktop.... OS X is way better for the very reasons given above about lack of integration which you know will never be there because of the way Linux distros work.