Pros of bringing your own seat:
- no extra cost, permissions, waiting while they find the seats at the rental agency
- you know how to work the seats
- your kids are familiar with the seats
- you can also use them on the airplane
Pros of renting a seat:
- the car will be sure to have tethers
- the seats are sure to fit the car and be right for the country you're in
- the seats might already be correctly installed in the car, possibly saving time
- you will have less to carry in the airport
If I was landing late at night somewhere that I didn't speak the language, or if my children were creatures of habit who didn't sleep well in unfamiliar situations, I'd bring the seats and control my own destiny. If I wanted to use the seats on the plane (having paid for kids tickets), I'd bring the seats. If I had complicated stroller/car seat adapty combos, and wanted to use those strollers at my destination, I'd bring the seats that worked with the strollers. If I hadn't bought plane tickets for the kids, had lots of time on landing and knew that my kids accepted unfamiliar things without much complaint, I'd rent the seats for maximum convenience and bring some soft flannel blankets to make random seats more appealing to them.
I don't know how long you're renting for, but in the overall budget of a flying vacation for four people, I wouldn't let 20 or 30 Euros factor into the decision at all. I would just be trying to make the day of flying and arriving, and the day of leaving, as easy as can be, and to keep my kids content.