It really depends on what you want to prioritise, there isn't a simple answer on what seat is the safest.
If you're looking for the best test results, you can look at ADAC or ÖAMTC however both of these sites are in German so can be a bit of a trial to search. My German is reasonable - I'll look some up for you.
(Smaller numbers are better.)
Maxi Cosi Rodifix XP scores 2.6 for safety. I don't think the Airprotect is tested.
Best scores just in safety category (bear in mind not every seat has been tested):
Britax Romer Kidfix III M - 1.6
Cybex Solution S-Fix - 1.7
Britax Romer Kidfix SL SICT - 1.7
Joie Traver - 1.8
Cybex Solution M (and other variants of M) - 1.8
Joie Trillo LX - 1.9
If you google ÖAMTC + seat name you can look at the breakdown (front crash, side crash, sturdiness of installation) or check whether a seat has been tested - many models haven't.
As your child is 3, you may wish to prioritise child seats with an anti-submarine strap, that is a strap that goes between the legs and hooks over the lap belt to stop them slipping under it. Britax Kidfix range have this in some of their models, as does the new Besafe i-size booster. I think a Joie one has it as well.
If she naps in the car at all, you might want to look at seats with a headrest designed to "catch" the child's head as they sleep. Cybex's headrests recline, Britax's are built in a V-shape - I don't know whether other brands have similar points.
Long term, obviously safest means the one you can use the longest. If you stop using it at age 9 and move to a backless booster for 3 years where you could have kept her in a high back, that's less safe. So you could go into a shop with a tape measure and see which has the highest back rest extendability. Diono seats claim to be usable up to 160cm, which is much higher than the UK recommendation.
Of course, safety also means a seat that fits your child well and fits your car well. Rear headrests can often cause a problem with high back boosters and need reversing or removing so that the seat can be flat against your seat back without a gap. If you have very moulded seats this may also cause a problem. So trying several seats in your car for a good fit is important. Also trying them out for a good fit on your child. Look for - lap belt crossing thighs or hips, not abdomen. Shoulder belt crossing collarbone. Head nicely contained in head rest. Comfortable for her to sit up, not slouch forwards (you may want to add a box as a foot rest especially as she has been rear facing).
Lastly - you do have the option to keep her in a harness until 25kg with the Joie Bold or even rear facing until 25kg (which is undoubtedly safer) with a seat like the Two Way Elite as mentioned or an Axkid seat, or a larger Besafe. Just thought worth a mention in case you didn't know :)