I'm planning on buying a new flattop upper for my bushmaster shorty...but the more I read the more confused I get. What kinda difference is there between a upper parts kit like Model1sales,JT distributing,etc versus say a upper from Bushmaster or RRA? Are we talking better mechanics or accuracy or fit or quality,or all of the above with name brand?
I appologize if this has been ask'd but im really confused, have read almost all the topics regarding this.

The quality of the brands you mentioned is about the same, pretty darn good! My experience has been its better to try and use parts made by the same company that your lower was made by because fit and finish tend to vary a little between most manufactures. Besides Bushmaster is one of the better companies in your list and thats who made your lower so I say deal with them. This is just my unbiased opinion, based on my experiences building AR15 rifles over the years.