Okay, if you did, I would wonder if it was a good idea at all as they really increase the number of air changes in a house to 3-6 times an hour. I follow a few builders who specialize in getting air changes per hour for a house down to under 1(Passive house:
https://passivehouse-international.org/index.php). This includes things like bringing the attic into the building envelope/conditioned space. Then you use something like this to provide filtered air into the house.
https://www.zehnderamerica.com/heat-...ilation-units/ These types of systems are great for someone who has allergies. So, an entirely different use case.
As far as single direct drive versus an older style fan, my assumption is that the older type is belt driven. This was likely due to the type of motors available and their costs at the time of installation. With a whole house fan, you want to be careful with how much air you are moving because moving too much at once could have negative consequences. So, I would suspect that a single direct drive has a few speeds which it can be set at to compensate/adjust volume and compensate for those negative consequences.