Checkmate and Metalform are Colt's two OEM suppliers and make almost all their magazines. They have done so for several decades. They make some of the best magazines available for the 1911. rbernie has clearly explained why the M9 comparison is a very faulty one to make concerning Checkmate magazines. Whether you get the long tapered GI type or the more modern Colt "Hybrid" type; either will work for your gun. I have never had a Colt/Checkmate/Metalform (or SA for that matter) magazine in good shape fail to properly feed ball into an properly built gun.
SA's factory magazines are Metalform, and also usually quite good. They often offer them to owners at very competitive prices (read: crazy cheap). You might call SA and see what they want for some SA mags, too. Another cost-effective option is that Metalfrom has their "five hole" magazines on a 1/2 price sale. Last item listed of their .45 magazines, last page of the (I think 4) they have of them. Click on "quantity discount."