As long as you're capable of following the instructions to fit the belt properly there's no difference. For the record we didn't scrimp on the seat, and got him a high end maxi cosi, but didn't see the sense in spending several thousand on a new car with isofix to save maybe 30 mins over the space of a year.
Said every parent ever in the history of car seats.
I'm not saying you aren't personally capable, however you are only human and mistakes happen. Be it because you are in a rush because you are running late or are just seriously tired because your little one kept you up all night.
There is a much much lower fitting error rate with isofix seats compared to belt seats. Especially with people who don't use cars seats day in day out I.e. grandparents. My dear old mother for example was never comfortable with fitting our first belted seat, however she has total confidence with our isofix seat as its just so much simpler to fit and even tells you that it is fitted correctly.