I have spoken to my wife about this, (she is a road safety officer and has a lot of experience with child car seats)
Isofix is essentially as safe as traditionally fitted car seats but with one fundamental difference - ease of fitting. Car seats are difficult to fit correctly, on the last car seat checking day my wife did, only 32% were fitted correctly! Therefore she recommends Isofix as there is less room for error in fitting.
As long as you choose a known brand you can't really go wrong, Britax are good, avoid Mamas and Papas like the plague, they barely meet the legal minimum standard. Which? wouldn't review them as they were deemed to be so poor. If you're having a non-isofix, try it in your car before you buy, seatbelts are all sorts of daft lengths and you need to make sure the seat fits snugly. If it's properly fitted you should be able to rock the car by rocking the car seat (some serious effort is needed to fit one snugly, involving knees and bodyweight)
Finally, it's a controversial idea in Britain but please consider leaving your baby rear-facing for as long as possible. When your baby is growing up you want them to do everything early - walk, speak, first tooth etc. but going forward facing in their car seat is the opposite, have a look at this website
http://www.rearfacing.co.uk/ The Scandinavian countries are world leaders in road safety and they widely advocate the use of rearward facing seats for the older kids.
Try ringing your local councils road safety team, as they often do car seat checking, and some offer a try before you buy service. Contact details should be on the roadsafetygb website.
Hope this helps
