It's generally accepted that the deck itself is the most important part of the equation, with the most influence on the sound quality. It's then followed bybthe arm, and then the cartridge. Based on that, I'd say a deck change is the best way to go. It's no good getting the best arm and cart if the deck itself is holding everything back.
I know there will be some that will disagree with this and say that it's the cart pulling the detail off the record, and that all the deck has to do is make the plastic go round, but finely engineered bearings and bearing housing, along with a correctly weighted platter, a vibration resistant plinth/chassis/sub chassis, and a power supply that supplies a smooth supply to the motor all play their part. Let's not forget the geometry of the deck as well.