The original .284 was developed by Winchester and wildcatters decided it would be even better at 6.5mm (or .260) than the original .284 (or 7mm) and so the wildcat 6.5 X 284 came into existance. Some early examples of Winchester .284 brass were of extremely high quality, and current batches aren't too bad either but take a little prep. There were various versions of the 6.5 X 284 wildcat, none however were standardized by way of SAAMI or CIP (Euro equivalent). And some fella well recognized in long range circles bought ALL of the remaining highest quality Winchester .284 white box brass.
Norma saw all of this, and possibly with a little elbow jogging by Bill Shehane, came out with the 6.5 X 284 Norma as a standard CIP chambering. The case dimensions are just a very little bit different from the original .284 Winchester case, probably to make it a "unique" chambering for lawyering purposes. From what I've heard the original Norma 6.5 X 284 brass was of exceptional quality but I have not been overly impressed by any that has passed my way in recent years.
Lapua makes brass for the 6.5 X 284 Norma CIP standard chambering. The Lapua brass I've had for 6.5 X 284 has been of excellent quality, so much so that I have had a reamer specifically spec'd for use with Lapua brass.
But back to the rub. While generally the 6.5 X 284 wildcat based on the original Winchester .284 case and the 6.5 X 284 Norma are very, very close, they are not identical. Depending on reamer design they may be interchangeable in a given chamber or not. Ballistically they are the same.
If I had to do it over again I'd still go with the 6.5 X 284 Norma chambering, simply because of the excellent Lapua brass. That and the fact that mine flat out shoots so very well.