I had a 2014 981 Boxster and now have a 718. I did really like the sound of the 981 when the RPM was above 5,000 RPM. Unfortunately very seldom in my daily driving did I get it above that RPM to enjoy that sound. That was also the RPM that I really noticed a great increase in acceleration. I initially drove my 718 with the sport mode off and it sounded pretty good to me. The last month I have had the sport mode engaged all the time and really like the sound and burble that I get. Sport mode raises the idle RPM by about 200 RPM and you very quickly get the extra power from the turbo. What I enjoy most in the 718 is that tremendous acceleration at very low RPM's, when driving in the city. The car just jumps when you touch the accelerator. I also checked the gas mileage on my daily 25 mile drive and it decreased from 26MPG to 25MPG but I also noticed that I am accelerating faster when starting from a stop because I enjoy that rush of acceleration and the sound. I installed the memory module so that the sport button and engine cutoff buttons stay in the same position they were in when I shut off the car. Without that mod the sport button always defaults to off and engine cutoff to engine cutoff when you shut off the car. You will really enjoy driving your 718 compared to a 981. Remember the 718 base gives you about the same performance as the 981S.
The Audi sounds better at startup, the Porsche sounds better at higher revs. Audi R and RS cars always sound amazing and loud on startup. My neighbors told me that they always knew when I was leaving my house because the R8 V10 Plus I used to own would wake them up. Of course that car sounded great throughout the RPM range.
There will be very few cars in the price range that will be faster and quicker than a TT-RS over the road. You will not feel as connected in the Audi as with the Porsche, and Audi steering feel doesn't compare to Porsche (for example, my Cayman GTS had far more steering precision than my R8). The TT-RS is a broadsword; the 718 S is a scalpel.
Someone choosing a sports car and saying that the sound is a main reason for them would be out of their mind. Exhaust sound is something very minor because considering the aftermarket support and even custom services, you can tailor that exhaust sound to anything you would like.