I have shot both the Kel-Tec Sub2000 and the Beretta Storm CX4. I thought I'd like the Sub2000, but it was flimsy and the noise of the bolt cycling was loud (even with hearing protection). I'm not saying it doesn't work as a firearm, it just didn't seem like it was in for the long haul with a decent round count. In contrast, the Storm seemed like a relatively solid gun. It was fun. It did what I needed it to do and it seemed like it would hold up over time.
As far as the reasons for seeing many used Storms for sale, I think that is a function of market share coupled with the narrow niche of a pistol caliber carbine. While the CX4 is great for indoor range use where you may be limited to pistol calibers, it's not the tactical tool Beretta is trying to paint it as. I think a lot of people got one and then decided they didn't want $500 tied up in an oversized pistol with limited application. With the Sub2000, I don't think you see a lot of used ones for sale simply because they're not that popular to begin with and their resale value is low. If you picked up a Sub2000 for $300 and then went to a gun store to sell it, the store would probably offer about $150. At that price, you might as well keep it.
There are pros and cons to both designs (for example, Kel-Tec was brilliant in making several versions of their guns to be compatible with several different brands of magazines). That said, you should probably invest some time in trying both before buying either. Good luck...