Baduy,
There is nothing wrong with the Eclipse BCS bottom feed and I believe you can use Badger or Paasche metal cups or bottle assemblies in it also as the bottom feed airbrushes use a tapered tube that you just press-fit the metal cups into. You better check into that because I am not absolutely sure if they fit the Iwata. The Eclipse and the Omni 5000 are both great airbrushes so the choice is yours. I kind of like the gravity feed airbrushes for models because you can mix your paint and pour it right into the gravity feed cup and paint.
You can just put a few drops of paint in there also if you want whereas with a bottom feed you need more paint for the tube to pick up. Gravity feed airbrushes also spray at lower pressures if needed, but that isn't much of a factor in model painting.
One thing about a gravity feed airbrush is that you have to keep it pointed down if you do not have a cap on the color cup, otherwise paint will spill out when you tip it too far.
Also remember that a gravity-feed model like the Omni 5000 has a 1/3 oz color cup which is fairly large so it will take some getting used to seeing where it is pointed as the cup obstructs the view slightly but it is not that big of a deal.
One other thing to consider is that if you are not really careful with the airbrush you will need needles and tips more often and the price for the Iwata needles is almost twice as much as a Thayer & Chandler or Badger needle and the Iwata tips are almost three times as much.
I personally use the T&C Vega 1000 gravity feed for model painting although my Vega 2000 and Omni 3000 bottom-feed airbrushes will work fine too. Let me add that I use my Omni 3000 and Vega 2000 for T-shirt airbrushing mainly and the Vega 1000 is my modeling airbrush.
Here is some of my airbrush artwork is you are interested:
http://myartworkgallery.com/mag/slides.asp?member_id=14
Mike
“Wisdom is the right use of knowledge. To know is not
to be wise. Many men know a great deal, and are all the greater fools
for it. There is no fool so great a fool as a knowing fool. But to know
how to use knowledge is to have wisdom. " Charles Spurgeon