Watling was better as a bat when then they were keeping, but McCullum was a better bat overall.
Watling as bat:
8 tests, 320 runs @ 28, 1 100 & 1 50
Watling as keeper:
67 tests, 3398 runs @ 39.05, 7 100s & 18 50s
Overall:
75 tests, 3790 runs @ 37.52, 8 100s & 19 50s
McCullum as bat:
49 tests, 3650 runs @ 42.94, 7 100s & 16 50s
McCullum as keeper:
52 tests, 2803 runs @ 34.18, 5 100s & 15 50s
Overall:
101 tests, 6453 runs @ 38.64, 12 100s & 31 50s
Almost all of Watling's runs were hard earned, whereas quite a few of McCullum's keeper runs were downhill skiing when the hard work had been done/blasting weak attacks like Bangladesh. McCullum's later career batting (when not keeping) with the daddy 100s was better than Watling however.