I've only done the I-35/I-135/US-81 route (not all at the same time, but all of them within the last few years). The roads are great, and US-81 is four lanes all the way up to York. The only thing I'll say is that the speed drops to 55 at several of the small towns/junctions in Nebraska, and that you would do well to heed those speed drops. Other than that, it's a fine route.
I've never done the US-281 option, but I imagine it would be much slower.