Matthew, both are identical except 51470 has a picatinny rail to attach accessories and 51479 doesn't. Other than that they're identical. They both have same G10 grips, same barrel length, both have an adjustable rear sights and fiber optic front, same ambidextrous safety, and both include 1-7 round magazine. So depends if you want to add accessories like a flashlight or laser. If you don't then I'd go with the cheaper 51479 option since you'll get the same pistol minus the accessories rail. Hope this helps.

Here's a bit more info on the Tac series.
The TAC series is Rock Island Armory's most versatile, all-around 1911 and includes precise engineering elements built with every tactical advantage and choice you could want. All TAC series 1911s include a full-length under-picatinny tactical rail to allow any choice of custom attachment. Skeletonized hammers and triggers with over travel stop provide adjustments important to anyone looking for a fully capable 1911. The TAC Ultra Compact introduces a full tactical rail. It is built with a 3.50" button rifled barrel supported by a full length guide rod and full dust cover. Includes a low profile adjustable rear sight and high visibility fiber optic front sight and is fitted with skeletonized hammer and trigger with adjustable over travel stop. The TAC Ultra is finished with a tough parkerzied matte coating and adds battle ready G10 tactical grips. True to all Rock Island 1911s, the TAC Ultra comes with a crisp factory 4- to 6-pound trigger to give you a great experience right out of the box.

The 51470 "Tac" series guns have a full-length dust cover with a rail section for lights/lasers while the "Rock" series 51479 has a shorter, smooth dust cover.