Is there a big difference in quality between the authentic and the usual jersey?
Well. I noticed people think that replica is much worse. It's not. I've got many replicas in my collection and a few auhtentic shirts but that difference isn't big or something.
There is a question what are you going to use it for?
If you gonna play regulary wearing it then go for authentic one. The main difference it's designed for playing footy as it has anti-sweat holes everywhere on the shirt and so on. (this years Juve home replica has them too). Some people say the material is different but after comparing mine shirts I found out they're the same 100% polyester.
Replica version is a fan version. If you either put it on from time to time just to walk around or on matchdays then take that. After all both of these are official things and you ain't getting shit by buying replica which is great option to me. Go one page back and check out pics of that shirt I just got.