Which Car?

I suspect you may get a more accurate answer if you provide more information: how many suitcases and of what size(s), and how many backpacks and of what size(s)? And will there also be carry-on bags that must be squeezed into the trunk? The latter would be important if you were planning to stop for meals or sightseeing in transit from one hotel to another. It's considered very risky to leave anything visible inside a car parked at a tourist destination.

Looks like the BMW X3 has 550 liters of boot space and the Ford Kuga has 456 liters of boot space, so the BMW has almost 100 liters more boot space.

A possible issue with the BMW is insurance coverage. If you plan to use credit card CDW, the BMW may or may not be covered. For example, Visa USA says, "...selected models of BMW, Mercedes-Benz, Cadillac, and Lincoln are covered." and "For questions about a specific vehicle, call the Benefit Administrator at 1-800-348-8472". You could also purchase insurance through the rental car company or from elsewhere.

Last time we traveled with 4 people, we got a mid size wagon--an Opel. The rear end had exactly room for 4 carry on rolling bags and a couple of big purses.
The trick is to minimize whatever each person carries.
I've found that in Europe and elsewhere, often what specific vehicle you reserve may or may not be available whenever you get to the rental office. They go more by class/size of vehicle.

Of course you're never assured to get a specific car, is the big problem!

Are you tied to only these two choices? Remember as has been pined out by previous responses, the further you deviate from the basic stick shift econo-box the less likely you are to get what you "reserved". If you are going to be three weeks or longer take a good hard look at a lease. One of the many advantages to a lease is that you will get the model you reserved and if there are any issues you will know all the details before you arrive. There will be no surprises at pick site. Both Auto-France and Renault USA have very complete descriptions of seating and luggage capacity.

You do not indicate luggage size, it covers a wide variety of sins. We travel with carry-ons other people think they can only travel with a huge suit case. The trick is to pack efficiently and take only the REAL necessities. When you are traveling far from home, nobody knows or particularly cares who you are and demeanor is far more important than appearance unless you are wear clown suits or 40 acres of spandex.

Thanks for all the advise. I will check out the cars here before I go and check with my credit cards to see about the CDW insurance.

As mentioned above, it is rare that you can request a specific car and get it. Usually, you can book a certain class and they give you samples of the kind of car you might expect.

We've traveled by car with 4 passengers frequently throughout Europe. We always request a station wagon type car. That usually gives you the best combination of luggage space and leg room for the passengers in the back seat.

We drive a BMW at home and while it is a nice car, the brand is notorious for its cramped leg room in the back seat :-( I would not rent it for travel with more than 3 passengers.