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Pretty much what Disco Dante said. If you want and plan on keeping a 416 get a 416; if you plan on changing anything get the M4. The 416 receiver's top rail is higher than an M4's and it'll make it a bitch to line up any aftermarket RIS/RAS front end properly. Ultimately you could buy a 2nd M4 upper but then whatever you saved up front getting a gun with the rail (416) you'll end up spending later getting the 2nd upper.
Since you seem to want to modify it at some point, I would strongly suggest the M4.
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