<p>Has anyone tried the steel-reinforced polyurethane timing belts? If so, how do they compare to the rubber ones?</p>

I do not believe the standard rubber/plastic belts have any significant stretching over time, nor do they stretch under drive motor force during acceleration (and in any case, extrusion takes place mostly under steady-velocity conditions).
While I suppose it's possible a steel-reinforced belt might have a longer lifetime, replacing a belt is quick and cheap, so why bother?
[incorporating comments]
My suspicion is that standard belts, e.g., as supplied with Prusa-clone kits will outlive us mere mortals. They can move an entire print bed+heater, so a larger print head is not an issue.
The usual problem with any belt material is ensuring there's no significant slack (which leads to backlash/hysteresis), and changing the material won't help or hurt tensioning control systems.