The gravity feed brushes are capable of slightly finer lines simply because they will feed at lower pressures. I have an Omni 3000 (siphon feed) that I've had for a good while and while doing the camo on my F-105 I kept running into feednig problems at very low pressure (not sure how low because my pressure gauge was barely registering, but somewhere around 5-7 psi). I decided to get the Omni 4000 (gravity feed) and it does feed properly at lower pressures.
This does make sense since in a gravity feed brush the paint is already present in the brush and doesn't have to be pulled up through the siphon tube.
The ability to switch colors by screwing one bottle off and another on is very handy, and it's one of the reasons I like siphon feed brushes, but at the same time you can always keep your pre-mixed paint in bottles and pour a little into the paint cup of a gravity feed brush.
Charger, you've picked three good brushes. I can't pick for you, and doing cars you probably won't need the fine-line capabilities as much as those of us who do things with multiple colors. Personally, based on my limited experience, I'd go with the Omni or the Iwata. Both will give you a lifetime of service, both will do anything you want.
Check your area and see if there are any dealers for Badger or Iwata. If so you have an outlet for emergency spare parts if you should happen to break something and need a quick replacement.