I know they've probably changed . . . but I remember a co-worker having a Kia Sephia from Korea (try saying that five times fast) and even though it had a nice, long warranty she had to buy several things for it over time . . . which is expected for some things such as the battery, but I remember the battery was pretty specifically sized to the Kia so she couldn't buy after-market at the time.
I like the looks of Mazdas and there is a lot of good write ups on Mazdas . . . but a Mazda 6 is still a far cry from a WRX . . . although they look mighty sharp to my eye. I suspect most folks cross shopping a Mazda 6 would also be looking at Honda Accords . . . maybe even Toyota Camrys.
You mention the size of the wheels on the WRX are an issue. Personally, where I live with lots of frost heaves, pot holes, etc. I like the 17s and never really saw a huge difference with the 18s . . . but as others have mentioned you can always buy 18 wheels . . . and if you live in an area that is snowy you will most likely want to get a second set of wheels and tires anyway with the WRX.
In your first post you mentioned some criteria:
Practicality: I suspect any of the cars listed are "practical" . . . although this is largely a subjective term. I will say having AWD for me with the WRX vs. front wheel drive or rear wheel drive was a "practical" selling point which was pretty significant for me. However, AWD and the often noticed decrease in gas mileage may be considered not practical for some folks who may never see snow.
Price: I suspect the WRX is the priciest of the three . . . but to be fair you're looking at something that even you admit is a bit different from the other two choices. As mentioned, Subaru cars -- particularly the WRX and STI -- tend to hold their values quite well.
Safety: Not sure how the others fair, but Subaru is pretty big on the safety aspect of their cars -- in fact I would say it is one of their major selling points.
Tech: Yeah . . . Subaru is most definitely not going to win this one . . . and it doesn't take much for just about any other car manufacturer to win this one. However, for me, tech was not high on my "must have" list . . . in fact I am just the opposite . . . I am more about the ride than the cool, "gee whiz" tech. I still have a flip phone -- which should tell you what I think about new tech.
Performance: WRX . . . all the way . . . when compared to the others you list. It is not the end-all, be-all car however like some folks (not here at this site) make it out to be. But it is a good performance car in all weather.
Handling: WRX. I ride sleds in the winter (well I try to . . . this past winter meant only two rides for me) and the WRX is the closest thing with four wheels to riding a sled.
Fun Factor: I've had my car for almost a year now. I still take it out every time I can . . . and in fact yesterday I purposefully drove a hour out of my way since I was enjoying the drive. I'm not sure you will get that same feeling with the other two cars you list.