When it comes to car racks, crossbars are the simplest part of the system but are critical to rack success. If you are interested finding the right bar for your needs, you're in the right place. If you're also interested in elementary metallurgy and physics, read on.
All crossbars are made of the same stuff - steel, clad in a vinyl skin. They vary in length but essentially they function the same - they hold up stuff. They are all made of A36 mild steel, hot-rolled and welded, which is basically scrap metal (old cars, etc.) melted and reformed. We haven't performed a metallurgical examination, it's just common sense -- if they were some sophisticated alloy, they'd cost $300 a piece. We know, we have a blacksmith steel junkie on staff.
Round vs. Rectangular
Yakima bars are round. They are round because Yakima likes the versatility of being able to rotate accessories on the bar to get them out of the way, for instance, to go into a garage.
Thule likes their bars rectangular for the exact opposite reason -- they don't want things to rotate.
Round bars are stronger structurally than a rectangle, especially if you start to pinch them (like the contact points of towers). Our engineering friends call this hoop strength, and it's good. The downside is that they provide just a tangential point of contact for sitting stuff on the bars.
If you have a domed roof (Audi TT, new Bug, or anything that looks like an arthropod) bear in mind that if you need accessories like a bike rack that require parallel bars, you may have trouble with rectangular bars.
Rectangular bars offer an I-Beam-like construction, which offers a nice flat surface for placing lumber and other such items. While they do not allow the accessories to rotate, sometimes you don't want them to rotate, right? Round bars sometimes let things rotate when they shouldn't, unless you install things correctly - which you should do anyway.
What about aerodynamic bars (or aerobars)? They're great. Upside: quiet, quiet, quiet. Downside: fewer accessories attach to them (for now). If your carrying needs are covered by aero-compatible accessories, and that's not likely to change, go for aero.