What makes the Polaris more than, "just a better Retaliator?
TL;DRThe Polaris differs from the Retaliator by carrying another ship in its hangar, supporting a full crew, and holding cargo without a module, with facilities supporting a broader range of contingencies.
The ability to have another ship in its hangar is the obvious one, plus support for a whole crew. Along with the ability to carry cargo without the need of a module, essentially meaning you do not lose any existing features for it. Other than the engineering post of the Retaliator, it only supports the pilot and a few gunners, the Polaris can let a whole strike team work together at maximum efficiency. Generally speaking, it fills a similar role, but on a much larger scale, and carries with it the presence of a capital ship as well as extra facilities that support the diverse skill-set of its crew which goes beyond piloting, gunnery, and the firing of torpedoes. The cargo hold can be used for mission-related supplies such as spacesuits, ammunition, repair supplies, medical equipment, and other things that make a sustained, more self-sufficient deployment possible without compromising its offensive capability, as well as prepare the Polaris for a broader range of contingencies than only torpedo delivery.