Should a power drill or impact driver be used to drive screws

Drills and impact drivers are similar in that they're both rotating power tools and they have some overlap in usage, but they work differently and have different strengths.

Drills apply a constant torque and tend to have good control across a range of speeds and clutch settings. They're good for work that requires care or precision, like drilling a hole or driving a a smaller screw.

Impact drivers provide much higher torque but in rapid bursts–very much like a hammer provides bursts of linear force. This makes them good for heavy-duty jobs like driving long lag bolts (you can even take the lug nuts off your car wheels with most impact drivers), but the banging action makes them a little bit difficult to control. They're also not good for driving into something very soft–between the hammer strikes there's no torque applied and the bit can actually rotate backwards. You can see this by holding the chuck with your hands while the driver is on; you'll feel the pulsing of the strikes but chuck won't actually rotate. Finally, some impact drivers don't have a clutch, which means you need to be very careful if you're driving something that could be over-tightened or broken by the high torque.

Besides the higher torque, impact drivers have another huge advantage in that you don't have to hold them tightly to provide the torque. The momentum of the rotating striker inside is what provides the rotation force, so even when you're driving something very tough it's quite easy to hold the tool. Drills on the other hand provide a constant torque and require the user to provide the reaction and prevent the tool from spinning.

If you have to pick a single all-purpose tool for general DIY use, a drill is much easier to control and is useful in a wider range of situations.

This isn't something I would worry about at all. If you are a typically householder you don't use your driver 8 hours a day 5 days a week year after year after year. If you do, you should be buying a tool designed for professional use (which the DCD790 appears to be, judging by it's price).

I'm just a householder and I find drills last me at least 10 years. I still have all the drills I ever bought and they all still work (to some extent). On one of them I can't turn off the hammer-action after trying brute force removal of chuck (I broke it by severe misuse), but the clutch is fine. I bought a pro-grade replacement drill/driver (Bosch blue) at a similar price I paid 10 years ago for the non-pro one I broke.

The clutch on the driver is essential, I always dial it down to the lowest setting, then turn it up as needed. I'm pretty sure it is designed to be used and shouldn't wear out any faster than the gearbox or motor.