Base/Clear is not really shinier, though you can get much better depth to the gloss. This depth can be impressive and when you see a car/plane really "pop" it is usually the clear depth that does it. However, that comes from numerous coats of clear, which you want to avoid for weight reasons on a plane. I put five coats of clear on my Cobra project and it looks gorgeous. That said, most single stage paints won't take much cut and buff, so that could impact "shinyness."
On the durability front, you'll find single-stage more durable. The weak link, from a durability standpoint, in base/clear is the base. The primer adheres very well, as does the clear coat. The base, on the other hand, does not have that strong of a grip. This is why you see more rock chips on modern base/clear painted car than you did on the old lacquer paint jobs. You'll notice that the chip takes out the clear and base (adhered well together) and leaves primer. This is due to the relatively weak bond between the base and primer, as well as the softness of the base coat.
Surface durability is similar and varies for either based upon how the manufacturer tweaks the polyurethane formula (both Clear and single-stage are polyurethane based - avoid the cheap Acrylic Singe-stage paint) and the additives used. They range from soft to hard, based upon the trade off in attributes they are looking for. Hardness helps in durability but makes buffing difficult. This should be considered for anybody painting themselves in a hanger or garage. UV protection comes from an additive and applies equally to both paint styles. All pigments are susceptible to sun fading and require UV protection.
Single-stage is really just clear coat with pigment mixed in therefore it has a strong grip with no weak link. However, only a small amount of pigment-free clear ends up at the top and you need to be cautious on cut and buff. Some paints, like what i used, use Polyurethane pigments and you can cut/buff into the pigments without impacting the shine.
The primary benefit of base/clear and why it is so commonly used today is that is more tolerant of user technique than other painting methods, especially for effects, such as pearls and metallics. That said, straight color SS's are really no harder than base/clear with the exception that runs are more problematic.
Larry