Kennedy's can be good for flavor and clouds both. They redesigned the deck, so the Atty's with old style decks are going cheap on
kennedyvapor.com ... The build your own Kennedy with Gremlin cap is $75.
Derringer is a neat little atty, but the post holes aren't the biggest, and to redrip you need to pull the topcap and realign the afc. There are aftermarket body's for derringers where the top cap is more like a bambino in the sense that you just pop the top leaving afc unaffected.
With a Kennedy, you can just pop the cap and put it back on any which way, unless it's a demon cap, without worrying about afc.
You gotta worry bout over dripping on any of these atty's, but more so on a Kennedy because of the bottom airflow. There's also a 2x difference between the Kennedy and the Kennedy Comp, with airflow. On a Kennedy/Kennedy Competition you can get really great flavor & clouds with the right coil/wicking set ups. I'd advise a Kennedy/Comp Kennedy if you're semi-experienced with RDA's already because you need to center the coils over the afc, wicking needs to be right, over dripping, etc, to prevent leaking.
An alternate to the derringer in terms of size would be something like a hobo v3.1, it's a 2 post design like the velocity but only 2 post holes. Its great on flavor and clouds. Also a Dinghy by Flawless, same size profile & made by the makers of the tugboat. There are a lot more low profile atty's on the market now.
As for the velocity, I can't really say, I've never had one, but I have built on them. Super easy to build on. Best option for single coil is one coil straight through the middle of the posts. I hear people rave about it and there's so much hype behind it. 2 post atty's are my personal favorite to build on (super easy) and I personally don't see how one could get a lot of leakage from it if the direct airflow is closed off.
In the end it all depends on what you build. All 3 atty's are capable of producing massive clouds with the right set ups. Flavor is always dependent on your wick and wire IMO.
I've been debating a Kennedy 24 vs a Hyon Pugio vs Aeolus v2 for a month and a half now, so don't rush your decision if you're buying authentic.