Difference between better rating equipment?

Solution 1:

You need to look at the rest of the specs.

I believe all of the kill warrant scanners have the same 10 second time, but the D has a 2.5km range and the C has a 3km range.

Here's a chart of all the specs, showing that the higher rated warrant scanners pull more power, get more range, and cost more: http://elite-dangerous.wikia.com/wiki/Kill_Warrant_Scanner

I think I started with an E kill warrant scanner and how have the C, and that extra scan range really helps.

Note that it's often true that the higher spec gear has a higher power draw. It basically means you can easily equip one or two higher spec items, but then end up needing to also upgrade your power plant in order to power any more. (you can also carefully set the priorities on your modules to squeeze in a tiny bit more stuff by letting things you don't need during combat to shut down when hardpoints deploy).