Well you trying to compare apples to oranges in my opinion. Regardless of which car you pick, is it going to be your daily driver or a weekend car? The B58 has only been around for 5 years or so and its just now with the introduction of the supra is everyone realizing that it can make great power. The EJ257 has been around for about 16yrs hear in the states, the aftermarket support is huge here for the sti. The EJ although flawed, has been proven to make power and hold its own over the years.
I personally think the sti is a great jack of all trades car in my opinion. The sti is still a turbo, 6MT, AWD sedan. What will you do with the supra, a small 2dr auto coupe in the winter and or year round?
I bought my 2019 STi over a WRX and other options for a few reasons. First off I appreciate the raw "racecar" feel, which almost every other car on the road has forgotten, with adding in too many electronic help me drive add-ons. I also appreciate a car that can do more than move fast in a straight line. With an STi you have exceptional handling and braking, an AWD system which is controllable via the DCCD, and a balanced platform with a low center of gravity. If you plan to modify, aftermarket support is huge, and lets be honest not much beats the sound of a subie boxer. For me its also my daily driver in central PA so I can enjoy my car year round with my extra set of wheels and winter tires and I have 4 doors for my two kids to easily get in and out of.
The STi is a car with racing aspirations thats also a functional and practical (sort of) daily driver.
Also I'd like to add that the Supra uses a BMW engine. So if budget is of any concern, I tend to stay away from German engineered anything. Been there... it can be costly and a serious pain to work on!
For the cost of the Supra, I would rather get an actual BMW as they are nicer than what your paying for the Supra. I've been inside of them and didn't care for the quality or the looks of the interior. Also can't compare as the rear seat are useless in the Supra and STi has plenty of room if you have kids or carry passengers.