The handle grip is a personal thing, so what works well for me may not for another. The Arius has the most comfortable handle of all my knives. It fits my XL hand perfectly, especially for a saber grip. My thumb nests naturally into the upscoping curve of the blade, with no need for jimping at all to make it feel locked into place. The F3R has my next favorite handle. It's slightly wider than the Arius (0.61 inches vs. 0.57 inches, respectively), and I like enough width for a substantial feel to the grip. For me the posterior handle swell of the Arius is just right. It feels more nimble in my hand than the F3R, whose longer blade feels just a bit more unwieldy in comparison.
They're both great knives. The Arius is my favorite knife. Aside from the handle, I love the blade. Its unique design is not for everyone, but its high hollow grind makes it pretty thin behind the edge for its size, and it feels very solid, with a tip that's reinforced by maintaining a lot of the blade height close to it. It's a little lighter than the F3R (4.15 oz vs. 4.65 oz, respectively), but both are plenty light relative to their size. The construction quality is outstanding in both knives. The Arius is simple to disassemble and put back together. I haven't tried that with the Shiro. The overall action of the Arius is the best in my collection. The Arius drops more freely but in a controlled fashion. It's equally snappy opening both with the flipper tab and the blade hole. The F3R has a slightly softer detent but still great action. Service from Koenig is a big part of the appeal of the Arius, really exceptional and responsive.
You mentioned the flipper tab. I like how the Airus's tab continues the sweep of the curve from the handle's finger groove. The latest version of the Arius has a smaller tab that many prefer. I actually like the old style, but with the size of my hand, my index finger comes down on top of the tab and not at the point from the side. It's like the tab on Sharp by Design knives, which I think is also polarizing and which I also really like.
The F3R was substantially more expensive for me, but some Arius configurations can get pretty expensive as well. The Shiro has a feeling of solidity to it, much more so that the HatiOn I had. I think this comes in part from the thick Ti liners under the thick CF scales. Both knives have a lot of little design touches that indicate plenty of thought from the user perspective went into them. The F3R is impressively thin behind the edge: 0.012 inches vs. 0.019 inches for the Arius. The blade is a more conservative but attractive design. The Arius can come in a variety of finishes, while the Shiro always has the same uniformly stonewashed finish as all production Shiros seem to have. I could go on about each of these great knives, but the short answer is that for me, the Arius is a little better, but the F3R is worth its price if any expensive knife is.