I have the Sig M400 and the Colt SP6920. IMO, the Sig is every bit the rifle the Colt is. Here's a quick comparison run-down:
Both have an F marked front sight base.
Both have M4 feed ramps cut before anodizing.
Both have 1:7 rifling.
Both have chromed chamber and bores.
Both have equal staking on the gas key and castle nut.
Colt has an M16 bolt carrier.
Sig has an enhanced AR15 bolt carrier.
Colt bolt is MP marked.
Sig bolt is not MP marked although Sig states their supplier MP tests all bolts, whether they are marked or not. *
Colt comes with an H buffer.
Sig comes with a standard carbine buffer.
Colt barrel is MP marked.
Sig barrel is not MP marked and Sig could not state if their barrel is MP tested. *
Colt barrel is HP tested.
Sig states all of their barrels are HP tested, whether or not marked. *
Colt barrel is button rifled.
Sig states their M400 barrel is cold hammer forged and made of chrome moly vanadium meeting mil-spec. *
Colt has a standard mag release.
Sig has an ambi mag release.
Sig has a upper/lower tightening device in the lower, the Colt does not.
Sig has built-in sling point attachments in the lower, the Colt does not.
Sig has an extractor support in the barrel extension, Colt does not.
Sig has 6-position mil-spec diameter buffer tube, Colt has 4-position mil-spec tube.
Both have equally bad single stage triggers.
Out of the box, the Sig had no blemishes or marks and was cosmetically perfect, if that is important to you. The Colt had various nicks and dings from careless assembly, most notably around the trigger guard where the rear pin was installed, but also on the delta ring and on various spots on the upper receiver.
The Sig upper/lower fit is tight. There is no play whatsoever - either in the vertical or horizontal. The pins can be pushed out with your fingers. There is slop in the Colt that can be fixed substantially with an Accuwedge.
Both consistently eject to the 3:45 position with everything from cheap PMC Bronze and UMC ammo to NATO M193 and M855.
I can discern no practical difference in accuracy.
Both have been 100% reliable when using quality ammo.
Sig has a lifetime warranty, Colt one year.
*I spoke to Sig Sauer directly to determine this information. With patience, and insisting on speaking to folks who can answer these questions with authority, they will speak to you about the parts used on their guns – although they will not tell you who makes their components.
Here's a comparison of BCGs, and pic of Sig castle nut staking: