Hi Fred. Welcome to the forum
All 8Y A3s with an S-Tronic gearbox are MHEV. Key advantages are:
1. The auto-stop-start system is very much smoother than conventional 12V systems (because the higher power motor in the MHEV can restart the engine instantly). So you can avoid pumping out pollutants when at a stand still without any inconvenience.
2. The MHEV allows the car to switch the engine off when coasting, which improves your mpg a little (and is more environmentally friendly). This is seamless, you can only really tell what the engine is doing by looking at the rev counter.
3. The MHEV uses regen under light braking, which improves fuel economy and reduces brake wear.
4. The MHEV uses the electric motor to fill in gaps in the engine’s torque curve. This improves the response of the 35 TFSI at low rpm (before the turbo has kicked in) like a low power supercharger.
5. In theory at least, the MHEV has a much bigger battery, which should reduce the chances of a flat battery. And I’m assuming that, because the load on the 12V battery is reduced, it should last longer than the one in the manual car and be cheaper to replace. (The 48V battery should last the life of the car.)
6. I haven’t tested this myself but, since the air con compressor is electric, the car can run the aircon with the engine off (eg in stand-still traffic.)
This all from my experience of owning the MHEV for a few weeks and a long interest in hybrid systems.