Firstly the canbus issues referred to are not an issue with the NX350h or NX450h+.
When I changed from my NX300H to the NX450h+ I described it as going from old school to leading edge. The car is in my opinion superior in every aspect, particularly in terms of infotainment and interior design. I also found it much more economical in that all my local journeys were completed in EV mode. After converting electricity costs (overnight tariff) to petrol costs I was achieving up to 73.66mpg against c41mpg average of the NX300h.
If the PCP deal is attractive enough I would recommend going for the 450h+ and if possible stretch to Premium Plus trim. You will also have the added benefit of the manufacturers 3 year initial warranty. Don't get me wrong- the 300h is still a fantastic car and if you can get a low mileage 2020/21 model with full history you won't be disappointed but personally I would go for the new 450h+.