I can't decide between a cmt chrome lined upper, or rra chrome molly upper.  Whats the difference, and which is best? I understand molly is harder and makes a good mg barrel, but the build will be a semi.  Thanks

It depends on what type of shooting you are doing. For a low-maintenance SHTF type rifle, chrome-lined is the way to go.

Chrome lined barrels won't be as accurate so I hear, but plenty accurate for most uses(including the military).

Chrome lined barrels do last longer, and are easier to clean by a long shot.

Chrome lining is easer to clean and the barrel will last longer, which is why it is used a lot in the military.
A chromed barrel will be slightly less accurate than a similar chrome-moly barrel, but most folks won't really notice.

if those are the choices, definitly chrome-lined

The question you are really asking is chrome-lined or unlined, not Chromoly or lined.  Most GI barrels ARE chrome-lined chromoly steel.  Chromoly (short for chromium-molybdenum) steel is an alloy that is not very well blended, which creates some drawbacks.  It is strong and takes its heat pretty well, but is highly prone to corrosion and uneven wear, which is why these barrels are usually chrome-lined. A bullet passing through an unlined chromoly barrel will pass over patches of chromium, then patches where the molybdenum is in higher concentration, etc.  Each of the metals that go into chromoly have different hardnesses and corrosion resistance, so this is why when chromoly burns out due to use it will have bare patches where the weaker metals have worn away faster than the other metals.  When your rounds start to keyhole on your target, it is usually a sign that you've worn a bare patch in the rifling of your chromoly barrel.

If longevity and volume of fire  is your thing, go with a chrome-lined chromoly steel barrel.  If accuracy is what you're after, then go with a 416 stainless barrel.  Stainless is about as perfectly blended as steel gets, so you won't get little bare spots in the barrel resulting in the telltale keyholing that happens to chromoly.  Stainless wears very evenly, so the rifling wear that might be visible at the muzzle will likely be the same along the whole length.  Chrome lining a chomoly steel barrel mimics this to a degree in that the internal surface of the barrel is the same along the whole length.  Chrome lining makes it slick and easier to clean as well as uniform.  The trouble is that chrome lining cannot be applied evenly inside the barrel, so accuracy suffers.

Hope this helps.