Go try 100 innings on each and report back. You can count successful run outs if you like instead of balls pocketed.Easier to make balls yes, not necessarily easier to run out.
On average you lose your turn at the table on the 9' because you missed.
While on the 7' on average you lose your turn at the table because you didn't get position or don't have a good/practical shot.
I did it with straight pool. That game has more clusters than any other. Not only did my total balls pocketed go way up, so did my high runs.