Get the Cybex Aton. An excellent seat with very good safety ratings and very easy to use.
I think we may have tried that one already when we had a brief look. Whatever it was we tried wouldn’t spin properly and the staff refused to help fit it for “legal reasons” which was handy :-/
If it was the Cybex you were looking at then I would strongly suggest going somewhere else.
a. It does spin properly, it goes a full 90 degrees from rearwards for getting out. To spin the full 180 degrees you simply need to tuck the straps away first as it uses straps when sitting rearwards and an impact bar when forward facing.
b. Go to a proper car seat shop, decent ones will help with fitting and have staff specifically trained to do so. Our local Mamas & Papas did so when we bought the Cybex but I also got fitting session at our local John Lewis and local independent to try different seats. If they are unwilling to do a fitting, I personally wouldn’t be willing to trust them to buy seats from!