May is the middle of the rain season on the Colorado Front Range. As you've already experienced, there's some big weather in this area. If you go north (I15 to I70) the interstates clear quickly after bad weather. If you go south through Zion and Durango, the ride is better and the scenery is fantastic however, the chance of bad weather is higher. Many of the higher passes still have snow on them in July. Personally, my choice would be to go through Zion and Durango, be prepared to lay over, enjoy the local restaurants, enjoy the ride and avoid the slab.
2015 RGS, Danny Gray, Ness Bars, Banana Boards, Bassani B4, Cams, Heads, Power Vision Remap, Chrome Enforcers and other stuff.