I've used both (own both kinds - both gravity feed and siphon feed), and I like my gravity feed better...but then, it's a better airbrush overall, so that may be coloring my preferences.
I've found the gravity feed works better with less air, so it's great for detail work.
The siphon feed, I actually kind of like for broader area applications...it makes a wonderfully fine mist, but takes a bit more air pressure to produce results.
Cleaning the gravity feed is 50x easier than the siphon...but then again, the brush is better designed in general.
(that, and I paid considerably more for it, even at a discount)
If she expects to change out the colors a lot, keep in mind the siphon feed bottles are fairly cheap, so having a number of them around won't be as expensive.
Does the gravity feed you're looking at even have interchangeable reservoirs for paint?
(mine does not - but cleaning it takes no time in-between colors)
I guess the bottom line is: you'll get what you pay for.
Oh, and keep in mind an in-line air filter with regulator will make her airbrushing much easier.