Both are great products. ARO is much cheaper and its focus is on the exterior, such as cleaning wheels, tires, wheel wells, engines, etc. I believe it is widely stated that ARO is a clear-coat-safe wheel cleaner, but I don't think you can just go around spraying it on your paint...it's safe for clear-coated WHEELS, not your actual paintwork. The dilution ratio for paint is 8oz per 5 gallons.
P21S TAW is a multi-purpose cleaner that focuses on multi-surface safety. It won't strip wax and you can spray it anywhere on the exterior of your vehicle and it will just clean it without or harming the surface - you do have to rinse it off though. And then you can dilute it 20:1 and take it inside to clean your whole interior. While P21S TAW can be used to clean wheels, tires, and wheel wells, I'd rather leave that really grimey stuff to ARO as it's even powerful enough to remove rust.