I'd avoid XCF for a couple reasons:
* limited import/export support outside of Gimp
* No editable vector or gradient layers (you can save paths and re-rasterize them to layers manually, but it's kinda awkward to, say, change a blue gradient to a red one with otherwise the same settings)
SVG is actually a pretty good choice; anything vector-y and things like colors and gradients can be modified at will; and you can still embed raster graphics if you need them via embedded PNG or JPEG.
And of course SVG is spoken by both standard free tools like Inkscape, and standard professional tools like Illustrator.
Not sure if Photoshop can export its vector/shape layers as native SVG, but if it can that'd be a win. Sounds like another trial installation in my future!