The leather is pretty soft and isofix holds the seats in pretty hard, I'd suggest using the seat belt with a thick towel between the seat and baby seat otherwise you'll have some pretty nasty indentations in the leather.
Even with seatbelts I was getting indentations in the leather, I had a blanket in the boot that I folded 4-thick and that made things better. ISO-fix is a great idea, we use it in all the more modern cars and it just feels more secure.