Is all copper pipe pretty much the same?
There are different grades and types of copper pipe that you of course should be aware of.  However I think your question is whether copper pipe is really a commodity type product - whether there's a difference between products of the same grade / type made by various manufacturers and sold by various retailers and wholesalers.  

This is a common question but it's a tough one.  

One the one hand, there are standards that apply to these products and any product that meets the standard should be adequate for the purpose.  That may lead you to conclude that standardization makes these products essentially commodities.  

On the other hand, big retailers press manufacturers very very hard for low pricing, and are not nearly as tough on quality;  DIY customers are not as demanding as the plumbing contractors, maintenance people, etc. that shop at plumbing supply houses catering to contractors.  Big box retailers are going to cater to their customers and their customers are primarily concerned with price.  

It's very possible that in some cases the same product from the same manufacturer is identical quality whether purchased in a big box store or in a specialty supply house.  It's also very possible that a brand may have to lower their quality (within acceptable standards) to win a spot in a big box store.  It's even possible that a manufacturer may make the same exact product to a higher standard for supply houses and to a lower standard for big box retailers.  

In my opinion, I have seen all three of these scenarios unfold over the last ten years as big box stores have grown and become more powerful in the market.