EU regulations are much stricter than US ones (various sources, including Consumer Reports) so you're better off with a seat that does comply with the R 44.03/44.04 regulation than one that's only to US standards.
Car seats secured with LATCH system (bottom two anchors, not the top tether) perform much worse than the same seats secured with seat belts (various sources, including Consumer Reports). If you have an IQ high enough to follow directions to attach a car seat using seat belts (and apparently many people don't, hence the existence of LATCH), you should only use seat belts and can lighten your seat by simply cutting off the separate LATCH anchors/strap.