probably not a lot
The 200 has a smaller wing and carries less payload but suppose take-off weight is 47 tonnes with 115 pax and it settles into long range cruise at M0.72 FL350 I'd expect it to burn off about 5700kg over a trip of 1000nm. The 300 has the larger wing, not as large as the NG and an appreciable fuselage stretch, hence the increased weight and payload. The take-off weight would be say six tonnes greater, i.e. about 53 tonnes with say 128 pax but the fuel burnoff would remain about the same thanks to more fuel efficient engines. Whereas the 200 would burn maybe 0.76 kg of fuel for each kg of thrust per hour the 300 would be more like 0.69kg/kg-hr. Flight time for both would be about two-and-a-half hours with the 200 just having the edge by a minute or so.