The CarSeatInfo site, which I think is quite well respected because it is not profit making or selling any car seats, think that seat belts are actually safer than Isofix in many situations.
The info below is copied and pasted from their site:
www.carseatinfo.co.uk/index.cfm?sid=19086&pid=292365
What is Isofix?
Isofix is a system inbuilt into the rear seats of many cars designed to "anchor" by way of metal brackets Isofix compatible child car seats. The idea is great except that to anchor anything properly the laws of physics requires three anchorage points and the Isofix system has only two! A third point is achieved by a secondary device such as a leg or tether strap but this is dependent on car. In practice therefore Isofix car seats are not the total answer though we do accept on occasion they can work very well. Isofix car seats often have great "crash test" results but these results are taken in the lab and are not accurate for all cars.
In "real life" we believe seat belts are often the best and here is the reason why. In an accident a modern seat belt is designed to stretch. The amount it stretches will vary and is dependent on numerous factors including kinetic energy.
As a very rough guide however if a driver do not wear a seat belt the force of hitting the cars steering wheel at 30 mph can be as high as 50 times their own body weight. A seat belt that do not stretch will reduce this figure to 30 times the drivers body weight but a seat belt that stretches reduces the figure still further to 20 times the drivers body weight.
And this is our point..... Isofix brackets DO NOT stretch, so whilst at first sight they may give great "crash test" results in the lab there is significant differences in testing with crash dummies than to actual accidents with real live children. There is one exclusion from this general statement. The "group 1" Jane Exo child car seat has a pneumatic shock absorption system to overcome these issues. (See Recommended Stage 2 Car Seats).
Consequently, we do not believe laboratory crash test results are the best way to gauge how good (or how bad) a car seat is. The most important factors are using the right seat and fitting it correctly.
We believe it is wrong of magazines to publish crash test results to members of the public who quite naturally do not understand the full facts about how the tests are (controversially) compiled. We do believe however video showing a bad seat is very useful as parents can see for themselves how a really bad seat performs.
A crash test result is very useful to industry professionals, but in our view only form part of what makes a great childs car seat.