Mil-spec vs commercial is used two different ways.
Size-
There are two types of buffer tubes (receiver extension tubes) on the market today. Commercial which is wide tube with cut threads and Mil-spec which is a thinner tube with rolled threads. You can have a cheap POC mil-spec tube or a High quality Commercial, but normally Mil-spec SIZED tubes are better quality, while the Commercial ones are POC. Remember this is dealing with SIZE not how or with what material the tube was made. If you stick with a good reputable source for Mil-spec you are generally getting the best quality, and now US military stocks fit your firearm..;) Just remember there are high quality commercial tubes out there.
Process-
Mil-spec / Commercial in relation to how it was built, fit and finish, and QA inspecting. This varies widely. I have seen Colt crap that shouldn't have been sold and I have seen Blackthorn gear stand up to abuse. You are generaly good to go by using the saying, "You get what you pay for". High quality manufacturing cost $$. QA inspecting, fit and finish and high grade equipment raises the price of gear. On a side note, no company makes a Colt the way Colt does. That is top secret info and is Colt property. The US military inspects Colt firearms for compliance using these standards, and Colt had a cow and shiat fit when the US Military released this information.
A Magpul China-doll stripped lower will cost you about $2000.00 and is NOT mil-spec. I just saw the thread on that lower which is why I mentioned it.
Small parts are hit and miss. Springs SHOULD be made out of a certain material, and pins, hammers and triggers are "you get what you pay for". You will pay twice as much for a LPK made by Colt then you will for a Stag LPK, but Stag is still good solid parts, if you buy an LPK off gun broker from a guy in China and the shipping cost more then the LPK dont expect the parts to last longer then a week.