Which aircraft you are on is at best loosely correlated with the quality of in flight entertainment system you will use.

Seats are purchased by the airlines, they are made by separate manufacturers like Recaro or Zodiac and not by the airframe manufacturer (Airbus or Boeing). While the seats obviously need to be compatible with the airframe, there are a wide variety of options and it is common for airlines to refresh old airframes with new seats.

IFE systems are not manufactured by the airframe or the seat manufacturers, they are made by companies like Panasonic. They obviously need to be compatible with the seat, and my understanding is that it is uncommon for airlines to update the IFE system without also replacing seats.

So, while a newer aircraft type typically will have a more up to date IFE system, there are plenty of old aircraft that have new IFE and seats retrofitted.