To be honest, they're both probably as strong as each other, at least where it matters. (see pic), although the gray part is probably a bit stronger, as it has more plastic supporting the bolt, but either way, in order to fail, the part would have to split at a layer.
The forces the belt will be putting on the idler are going to try and pull that bolt sideways out of the part, so the most important part is that the walls that are keeping the bolt in position are strong enough.
So long as you print with enough perimeters, probably a minimum of 3, and high infill 30-50%, it will be more than strong enough. and if you print in PLA instead of ABS, the layer bonds shouldn't be a problem. (so long as you print hot enough that the layers melt to one another).