I did the research and bought one of those Ford isofix brackets when my 4 year old was born with the intention to see if would fit in my 2003 Elite saloon but never got round to it.
The isofix mounts fix the child seat much more securely than a belt - having used both types a lot over the past 4.5 years I am still unsure which is 'better'. The isofix seats we have won't stop beeping until safely clipped in whereas the belted ones can be insecurely fitted without it being obvious. None are actually easy to fit but the isofix ones do have bases that stay in the car which allowed is to quickly remove the baby seat and carry it into the house with sleeping baby in the seat.
In a crash the rigid fixings will accelerate/decelerate the child seat as quickly as the bodyshell but the belted seat will benefit from some cushioning from the belt and seat cushions