Car rental choice

Hi all,
My partner and I are planning a trip to Iceland from September 5-20, the idea is to camp for ~5 nights and stay in hotels for the other 8-9 nights. There are two options for the car rental:

• To rent a 4x4 with a rooftop tent
• To rent a 4x4 that converts into a sleeping space inside (like a dacia duster or subaru)

Both don't have heaters and 4x4 is needed for exploring highlands. Which one would you recommend? Are the winds intense around this time?
PS: We did look at 4x4 campers, but they are super expensive, considering we would only camp for few nights.

Any input is appreciated, thank you