the interior of the new one is so much better. I had a 2013 328i... and same interior. For the infotainment and interior alone I'd get the 2019. Its peak torque is only down 40 lb/ft too, and it won't be the "old" model. save fuel, car looks way way way more up to date, way nicer interior, better infotainment with full LCD cockpit, laser lights as option, remote start, wireless charging all the new interior goodies.
The 2018 340i other than engine/exhaust note and i suppose maybe more reliable since its a known platform is worse in basically every other way. The 2018 will have a non sport suspension as well since its xdrive unless you can find one with adaptive suspension on it (the G20, all the AWD cars have sport suspensions just like the RWD cars, which the F30 generation did not have unless you got adaptive) and won't handle as well. Also will weigh ~250-300lbs more