My theory is that O'Reilly was the better bowler per se
, in that his style and method of bowling could take wickets against everyone. There's an anecdote on Maurice Leyland's Cricinfo profile that's pretty telling:
If he knew himself to have the measure of the great O'Reilly, who was no paper tiger, he also retained the respect of one master for another. Describing an over of fearsome hostility, he said: "First he bowled me an off-break, then he bowled me a leg-break; then his googly, then a bumper, then one that went with his arm . . . ."
"But that's only five, Maurice. What about the last one ?"
"Oh, that," said Maurice deprecatingly. "That was a straight 'un and it bowled me."
Grimmett, on the other hand, had the multitude of variations - he was a mystery spinner - meaning he could absolutely tear through weaker sides, but he had a tendency to be found out a little against top opposition (not unlike an Ajantha Mendis or Sunil Narine). O'Reilly was the more consistent of the two.
I absolutely love Grimmett and his six variations of the flipper, but O'Reilly was the more successful bowler overall.