You ask a tough question. Let me try and answer it generally first, then give you my ideas on your situation later.
First off, it makes zero sense to own a gun you cannot adequately supply with air. So you should to start, know what amount of air you can reliably supply. Also you need to make sure you filter out any moisture. A compressor constantly running will supply heated air, and heated air will have moisture that is nigh on impossible to separate. So first off, AIR and moisture separation.
Secondly, the kind of gun will of course, be dictated by the project. Just as you wouldn't paint a plastic model airplane with a full size gun, neither would you paint a car with an airbrush. The detail gun is in between but far closer to the full size gun than the airbrush by a long shot. In a perfect world, you'd own all 3. Over the years I've acquired 7 or 8 guns of various types and sizes, and still use most of them at times. The problems in using a gun that is too big is control (applying too much paint or having to work too quickly to do the project justice.) While there is more overspray with a bigger gun, this is less a problem on HVLP setups, and I'd not rank that high on my list of worries - just mask well. The problems of using a gun that is too small is excessive time to apply the paint, requiring multiple passes and often resulting in "tiger stripes", also you may be refilling a lot, and on some paints... they dry quickly and you like to be shooting into a wet "edge".
If you can only own ONE gun, and didn't plan to ever paint cars or even full panels, and had just told me what you have, I'd probably opt for the detail gun. Between the fluid adjustments and pattern adjustments you can pretty much go from a small area of application up to to a full fan that would work for larger items. In fact, you COULD paint a car with it - I'd not want to watch that agony, but it would be possible.
The other benefit of the detail gun is that it would nicely model a bigger gun in terms of controls and practices - using far less paint. Play with it. get a big piece of cardboard and some cheap paint and fool around with it, exploring the controls etc... only way to learn. Practice your technique - speed/distance, evenness of both through the stroke, etc.
I hope this was useful.
