I don't understand why the other resplier (Ah-keong) keeps talking about what he personally prefers or like than trying to help the original poster in making his decision! OK, I will try my best to be unbiased here; lol.
I own the Sony system and I really like it a lot. However, I think one reason that makes the Sony system preferable is its relatively compact size and that it's overall lighter than equivalent Canon and Nikon systems. For a lot of people the difference makes or break a deal. Another win card of the Sony system is the superiory of their sensors comparing to Canon sensors though I have read the most recent Canon models have narrowed the gap.
However, they are potential disadvantages: the Sony mirrorless system is still and overall slower system compared to Canon and Nikon systems that have been around for decades and evolved to be fast: the camera's start up is slower, the AF may not be as fast, etc.. However, the third generation Sony APSC cameras (A6300, A6500) and the second generation Sony FF cameras (A7rii, A7sii) have narrowed the gap. I wouldn't be surprised if the next generation be on par with well established DSLR's.
Regarding AF system: the top of the line Canon and Nikon cameras may still have superior AF system (faster and more reliable). However, the lower end models may not be that good or any different than the most Sony cameras. So decision must be based on case by case comparison and not jumping on a general conclusion.
It's true that the Sony lenses are sometimes more expensive than equivalent C/N lenses. However, I have to say some of these Sony lenses are indeed some of the best lenses ever made. Just check the DXO tests and user reviews: Sony Zeiss Sonnar 24mm f1.8 (APSC), Sony Zeiss FE 55mm f1.8, Zeiss Planar 50mm f1.5, Zeiss Distagon 35mm f1.4, 90mm f2.8 OSS G macro, etc.. Then there are also excellent Zeiss Loxia and Batis lenses. The equivalent Zeiss lenses in C/N mounts may cost a lot more (Otus).
Then there are numerous perhaps less important differences here and there: OVF vs EVF (I don't have any problem with either one), features, user interface and ergonomics, available accessories, etc.. Do they make a difference in practice? Maybe; it depends on the individual. I like the fact that that most buttons on my Sony camera can be customized. I have programmed them to the settings that I use most. Do Canon and Nikon mid level cameras have this feature to this extent? I like the small and compact size but some people prefer bulkier and heavier cameras. A compact mirrorless camera like A6000 doesn't look as intimidating as big black DSLR on the street to the passing people. I don't get tired of carrying it around for long distances as much as I used to get tired when I had a Nikon system (D300, etc.). A less exhausted photographer has better chance of catching the good composition when it comes around.
Drawbacks: the A6000's AF is not as good and reliable as the Nikon D300 I used to have, particularly in low light. I hope Sony will soon overcome the gap between in regards to AF. Fortunately, the focus peaking and focus magnifier function help a lot in manual focusing.
So at then end, you need to decide what's important to you that makes you want to make the switch. Keep in mind that when you switch the system, the camera interface changes and you may need sometime to get used to the new interface.