put some effort into it, "search" the threads--you will find MANY topics that review these same models and diff opinions and ideas on em
rough opinion:
Stags and RRAs are the best ARs for the buck; DPMS is also another one, but i'd take the CMT and RRA over em (they simply have better quality imo)
the SW 15t is a great deal for what you get in the package, h/w, its noticibly more than the other 3 models you listed...so i guess, do you want the basic features of an AR or a more tricked out one like the SW 15T?
accuracy wise, they will be comparable and roughly the same--they will get you at least 2MOA at 100; reliability wise, they are near comparable...but like i said, i would put CMT/Stag (and since SW gets thier stuff from the same company) and RRA over DPMS
DPMS and RRA are more commercial vs Stag/and SW being more "mil-spec"...if that means anything
as to building, again, try a search, b/c this question gets covered almost every other day...imo, its cheaper to build IF you plan on building more than one or want to have more customized features, use an AR kit or complete upper/lower snap together, or get parts used
other than that h/w, it can be better to get an AR complete out of the box; say for example for new folks to ARs (or those that dont want to research/learn on how to build or invest in tools/time in it, etc), for warranty purposes and product service, or simply its "available" out of the box, ready to go
good luck