Just IMO, but the top securing strap only helps when the seat itself has its own built in restraint. Once a child is older enough to be secured by the car's seat belt, ISOFIX makes much more sense to me.
I got my niece an ISOFIX Recaro (Monza model that ISOFIX built in) for my folks car, and my sister is satisfied enough with the quality and security that I'm now getting another for her car.
My 2 cents, despite me not having kids.
