The Omni 6000 is a side-feed as you mentioned and it acts pretty much the same as a gravity-feed, although technically it is really a siphon-feed.
The main reason for the side-feed model is to give the artist a clearer view of what they are painting and that is why they were designed that way.
There is really very little difference other than the placement of the cup between the side-feed and the gravity-feed models.
The gravity-feed is slightly faster to clean because there is no siphon tube on the color cup to clean as there is on the side-feed, but the difference is not really worth mentioning. All three types of double-action, internal mix airbrushes are not hard to clean and take very little time to do once you get the hang of it.
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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