Regardless of F10 or G30, if you flip 530d upside down and rotate it the other way, you'll get the same outcome (IYKYK) 🤣 🤣.
Jokes aside,
2015 F10 would mean its an LCI and it has the N57 motor which is extremely solid. The F10 still looks really good for its age, an LCI is very hard to come by. Mileage and price ain't bad at 90k and R350k seems like a steal, considering F30 330d's around the same age are still selling for over R400k with more or less the same mileage.
The G30 has the newer B57 motor which replaced the N57, also a very reliable engine. The power difference is only about 6kw or so. Of course being a G series car, it will be a bigger step up in terms of interior and tech for sure. I personally find G series cars to be more comfortable and refined in comparison to the F series cars.
The 200k difference is a lot of money between the 2 cars. A decision like this all depends on what you looking for in a car and the type of person you are. If you someone that likes having the latest tech and gadgets, hands down go the G30. But if you someone that prefers raw driving experience, I would say go for the F10. Maintenance wise, they both will be out of warranty, and being similar platforms, the costs will be more or less the same either way.
If I was making this decision, I'd personally go for the F10, especially an LCI model (I might be biased towards F Series Cars). Not that I'm not tech savy, but I prefer the way F series cars handle and feel compared to G series cars. I recently test drove a G20 320i LCI as a potential replacement to my 2018 F30 320i, and yes its got all the latest tech, the nicer big screens, apple car play, very comfortable and quiet ride, but I felt it to be a bit bland and less engaging in comparison to the F30, which is what has made me hold onto my F30. And even if I wanted something like carplay, there are solutions out there. As I said, R200k difference is a lot of money between the two. My F series bias aside, I would rather buy the F10 for R350k, and save that R200k for any potential maintenance or mods.
If we were comparing F10 M5 and F90 M5, or even F8X and G8X, then my perspective would have been completely different despite my love for F Series cars.
All comes down to you at the end of the day bud. You can never go wrong with either option, both amazing cars.