Daniel,
Another thing you will like is that the Aztek side-feed cups fit in the Iwata side-feed airbrush so you already have cups that you can use.
They may fit the Omni 6000 also, though I am not sure.
The Aztek cups are easy to clean and are much cheaper than the Iwata cups too.
Plum1030's post about the differences in the HP-C and the HP-CS are correct, the main difference is the newly designed handle for quicker needle access, although these are gravity-feed and not side-feed models.
If you are spraying enamels than I don't recommend the HP-C line as they are too fine of a needle and tip for enamels. They work pretty well with thin acrylics though.
I personally prefer the T&C Omni airbrushes and they make a side feed which is called the model 6000.
The T&C Omni costs $89.50 U.S. and the Iwata HP-SBS costs $96.00 U.S. from Coast Airbrush.
http://coastairbrush.com/
Neither of these comes with an airhose though so that will cost extra.
Either one would be a good choice for a side-feed model.
The Iwata does not come with a Teflon needle bearing so if you want one it will add another $14.95 to the cost. The Teflon needle bearing is not really even needed though unless you plan on putting urethanes through the airbrush someday.
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