They are similar in overall performance.
The 6000 is a side-feed which is actually a siphon-feed design and some people like them. I personally have never used a side-feed airbrush and don't think I would really like it as it would feel odd with the color cup off to the side when all I have ever used are gravity-feed models and siphon-feeds with the bottle on the bottom.
Either one will be fine for you so the choice is yours.
The Omni 4000 may be slightly easier to clean because the color cup is mounted to the airbrush, but the cleaning time is not that great of a difference.
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