300ER,
Having operated both for some time I'd say the 737-800 is superior in its climb capability. It can go straight to 370 with virtually a full load and will make 410 after about 2 hrs cruise. The 320 struggles to make 330/350 when full. Also, the fact that the NG can get up to 410 can be a huge help, especially here in Europe, in getting direct clearances which can really cut time off your sectors. Of course, the 320 beats the 737 hands down when it comes to cockpit comfort - plenty of room to put things, properly designed spaces etc.