There are several big differences between an authentic helmet and a replica. The main issue is logically the quality of materials.
Helmet: High strength polycarbonate vs. less durable ABS plastic...
An authentic football jersey will have all if not most of materials (numbers, names, any other titles) sown on by hand or machine. Replicas have them painted on. Another difference is the authentic...
The "authentic" jersey, if it really is authentic, is the same as what the actual players wear in the games. The replicas are just that. They are not meant to be used on the field. Are less durable....