For me, I wanted a short barreled AR to actually be SHORT, not just in the legal definition, but in actual length, so I went with 7.5in on my first AR pistol. I recently picked up a 12inch upper because it was on sale and I wanted to try something in between the 7.5in and a regular 16inch. I haven't had a chance to shoot the 12in much so not sure how well I like it compared to the others. But in setting the 7.5, 12, and 16 side by side, the 12 really doesn't look that much different than the 16. But that's just me. I have a feeling the 7.5 and the 12 will be the two I shoot the most over the 16 though.