I would go with the darkest tint legally allowed in your location. I've never heard of anybody being pulled over or ticketed for illegally tinted windows, but chose to error on the conservative side to avoid giving a LEO an excuse to give me crap. Every state has different rules on tint %, location of tint and color (e.g. mirrored) tint.
Cosmetically (how it looks on the car), is a personal choice . Functionally ....
Darker tint will generally provide better UV, heat and infrared protection for the interior.
Visibility from inside to outside will not be noticeably different with a 10% VLT difference.
I tinted all windows except front, sunroof and mirrors with
Xpel Prime XR Black 35 (VLT 36%).
It's illegal to tint front window below
AS-1 line in my state. Front windshield is already embedded tinted from factory at top of front window and I didn't want a partial tint on front window. It would likely collect dirt, looks ugly and prone to peeling with exposed edge.
Things to consider ....
C1) Check you states tint laws.
Window Tint Laws | Car Tinting Laws
C2) Some tint has metallic fibers embedded which may impact AM/FM and cellular reception.
C3) Find a good installer. The functionality and longevity is highly dependent upon the installation. They can advise you on local tinting laws and should provide you a certificate indicating the tint is legal in your state/region. (Store this in your glove box).
C4) Avoid harsh chemicals and abrasives (ammonia, isopropyl alcohol) when cleaning installed tint.
C5) Lower quality/cheap tint will become purplish/brown over time. Spend the extra $ and buy good quality tint.
I used
Xpel Prime XR Black 35 (VLT 36%). on my 2011 Hyundai Genesis 3.8. Tint looked great at 100K miles. Same tint is on my current 2018 G80 with 23K miles 1.5 years old and looks like it was installed yesterday.