Sweet! Thanks for the advice, everyone.
We're leaning toward St. George, just because that is likely to be hot. My gf teaches skiing, so we're ready for warm weather. Moab does look great, though it's a little cooler than St. George by virtue of the altitude. With a warm weather forecast, we'd take it over the longer drive to St. George. We love to mountain bike, so either way we'll rent bikes and get out there.