You don't "have" to run a sand filter longer, only if you want it to remove the same debris as a DE filter. But even a longer run time may not match the filtering capability of a DE filter. Here is a fairly balanced comparison of each filter type: Filter Types

But to me filter run time is more about the circulation of the water than it is about filtering which is mostly for cosmetics. Keeping FC circulated during full sun and the heat of the day is far more important than removing every speck of dirt. On the other hand, it is very cheap to run the Intelliflo at low speeds so the extra run time would not be a big deal either. Solar is what will really negate some of the costs savings of the Intelliflo. But that is dependent on the details of your solar installation. If you do decide to go with a sand filter, just make sure you oversize it some to reduce backwashing frequency.

Also, is the the Intelliflo you are looking at have a built in timer? Most of the rebates require a controller, built in or external.