I always assumed that more grains = softer = more pingtasticness, but it’s simply not true.
More grains = looks more expensive and looks like the bats you see being used on the telly.
In my experience there's little if any difference in the performance of a bat, be it wide or narrow grain. But, in reality, narrow grain bats should be more expensive simply because the trees have taken so much longer to grow. There's a lot more care, attention and maintenance required for narrow grain trees, due to the time they spend in the ground.
For the farmer though, there's probably more money in wide grain trees, as they can fell and replant over a much shorter cycle.