I will always advocate purchasing a factory rifle that is equipped out of the box with all the features you want. If your budget allows for it, go for it.
1. M&P Sport, I can get one for $550,
but I know I'm going to want some magpul accessories
IMO, do not buy the 15-Sport
You'll get individuals who will advise you it is cheaper to get a sport, then deck it out in Magpul accessories. The issue is that most individuals are looking for the lowest possible cost for Magpul accessories. This short-circuits the critical thinking centers of the brain. There are tons of Magpul knock-off parts on the internet. It's so bad that I am very hesitant to buy Magpul gear unless its from a physical store or one of the big firearms specific internet retailers (Brownells / Midway). I got burned twice by counterfeit Magpul accessories.
Buy a rifle equipped with the Magpul parts from the factory. It will save you time and headaches.
2. S&W15 MOE Mid, has accessories and 1/8th 5r barrel. Can get for $1035.
3. Colt LE6920 magpul edition, for $1167 Thanks for any advise.
This is where you have to make the choice. The differences that stand out in my mind between the two.
Mid-Length Gas System v.s. Carbine Length Gas System
1:7 6 groove RH twist chrome lined barrel v.s. 1:8 5R progressive gain twist melonite treated barrel
There's a third thing that will come up: S&W brand image v.s. Colt brand image. The post quoted below is a prime example.
Colt. Pretty much the industry standard, built right, & battle proven. And it will probably never loose it's value as quickly as the others when you're ready to upgrade.
^ It has been my experience that Colt's brand name and brand image breeds this sort of knee-jerk reply. I am not implying that Colt makes a bad rifle, but they've got some AWB hold-over brand image from the "LE" branding. If it was really intended for only Law Enforcement (LE), then it wouldn't be sold to the general public. The Colt AR-15 has a superior brand image than the S&W M&P-15.
If you are image conscious and get butt hurt on internet forums easily, buy the Colt LE6920. The Colt LE6920 is a good rifle.
If you aren't easily butt hurt, then make an informed decision based upon the features of the rifle.
Carbine length gas system = proven to work. Has more perceived recoil impulse because of the greater pressures of shorter gas system.
Mid-length gas system = the new "it" thing. Has slightly less perceived recoil impulse because gas is tapped off farther down the barrel. The real advantage of the mid-length gas system is you get a longer handguard and a longer iron-sight radius.
1:8 5R vs 1:7 rifling = No big deal. Most people won't ever eke out the performance difference between the barrels.
Melonite = a hot salt bath nitriding metal treatment that alters the metallic properties of the barrel itself. It imparts hardness, heat, wear, and corrosion resistance similar to chrome lining. It penetrates the metal to a depth greater than the thickness of applied chrome lining. Accuracy potential of the melonite treated barrel is not affected because no additional materials are added to the barrel.
Chrome lining = hard chrome is added to the interior (chamber & bore) of the barrel. Hard chrome is hard. It imparts heat, wear, and corrosion resistance. When the barrel is bored and rifled it is done with more material taken out to compensate for the chorme lining process. Theoretically, a chrome lined barrel has less potential for accuracy than a non-lined barrel. Chrome lining is the tried and true mil-spec.
Mil-spec. True mil-spec is a set of standards set by military procurement to set a minimum standard for a rifle to be constructed for the U.S. armed forces. Mil-spec covers select fire rifles (full auto / 3-round burst) not available to civilians. Colt owns the rights to the TDP. It is assumed that Colt builds the semi-auto, civilian market LE6020 to as much of the TDP as possible. The TDP is a trade secret, so no one (one of us internet gun forum enthusiast nut-jobs) really knows all the details of the TDP to verify a rifle is built to the TDP.
Not Mil-Spec. Because Colt and H&K produce rifles for the U.S. Military, I can make the unverifiable assumption that those two brands have a higher likelyhood of being "mil-spec". Therefore the M&P 15 rifle isn't mil-spec. Remember, that as civilians we can take advantage of advancements made possible by civilian free market competition. Capitalism at it's finest.
So again this is all my opinion, you're going to have to do some research of your own. I would have a hard time choosing between the two. Part of me would get the warm fuzzies of owning a Colt and finally being able to sit at the cool kid table at lunch. I'd get to stare at the prancing pony and delude myself into thinking that the prancing pony itself holds depreciation at bay.
The practical part of me wants the longer handguard, longer sight radius, and less perceived recoil of the mid-length gas system of the S&W Midlength MOE. I also want the Magpul Art of the Tactical Carbine DVD's so I can get my tactical jollies off of watching the dynamic duo of Chris Costa and Travis Haley doing all sorts of cool "operater" moves & such.
S&W MOE or Colt MOE ... either one you'll get a good MOE-ed out rifle.
The day they started selling them in Walmart, they dropped value, as they were about $300 less than what they were selling for at the LGS. Then, they dropped the carry handle, the sling, the cleaning kit, the Rogers stock, and the 2 GI 20 round magazines in favor of a polymer Magpul BUS, Magpul 30 round magazine, and standard stock... lessening the value even more.
^Preach on brother.
Point this out to a Colt fan-boy and watch the butt hurt intensify. Then give them a double whammy by asking them to provide a credible copy of the mil-spec TDP so that claims to being mil-spec can be independently verified. Mil-spec TDP is Colt's secret sauce, and they don't want anyone to know exactly what it is.
Not all Colt owners are fan-boys. S&W has it's fan-boys too. Fan-boys (no matter the brand) walk around with blinders on. Can't stand em. Glock fan-boys are the worst...