I normally run I-43 down to Beloit, then head south on I-39/90 to Rochelle and pick up I-88 until it blends with I-80 just before the quad cities. Then I-80 west to Des Moines.
However, I've run my Suncruiser west on I-94 to Madison, skirted around the south end of Madison and taken US-18 to Prairie du Chien. US-18 is a decent 2 lane blacktop road. However, during winter (that's today up here
) it tends to drift because there's gentle slopes and curves and lots of open fields. Once winter is officially gone (2nd week in July) it's not a problem.
I haven't personally taken US-151 in Iowa. It's a good road (similar to US-18) in Wisconsin but it looks like there are lots of little "burgs" on the way to Cedar Rapids. Plus you have to filter through downtown Dubuque so it may cut down on the time you make.