Well, first question I would ask in my shop if I see you are already a vaper or not is where do you want to be? Simple and straight forward then the D30, especially as say a work or at home mod/unit, just looking for a step up but a little more involved, then the RX75. The D30 everything is included in the kit, battery unit, tank, charging cable, extra coil head, just need juice, making a great starter kit and/or secondary unit/device. The RX75 allows for more fine tuning, both can use whatever tank you want as well as Variable Wattage, but the RX75 allows better fine tuning in 0.1 watt increments, has support for Variable Temperature Control, so a little more advanced, downside but also a plus side, on top of the kit you'll need buy extra batteries, at least 2 (1 to charge while one is in use) and an external charger is highly advised if you do not already have these, but making it more expensive that way, it is a larger plus on the plus side over the D30, battery dies and ends its life cycle, you just need buy a new battery, mod is still good, the D30 being the battery is internal and not replacable, battery goes out the mod is useless afterward.
Today if I was a to buy a new mod in these situations
Beginner Just Getting into vaping -> D30
Beginner with a device already but needing a reliable secondary unit -> D30
Beginner to intermediate level wanting a second device and one that will advance with me relegating my existing device to a secondary role -> RX75
Intermediate to advanced level wanting a secondary backup device -> RX75
If you are wanting just a new device/unit, have the batteries/charger already I'd go the RX75 anyway, on a unit that only has an internal battery already then the D30, in the end it is still breaking down to what you as the end user are needing/wanting; others might chime in, but you need to ask yourself the questions of your needs/wants in a new unit, current vaper or just getting in to help narrow down on the strengths and weaknesses of each device/kit, what extras you'll need to/not to purchase, etc.
HTH