Going south to El Paso and then east adds 500 miles compared to going east to St Louis and then SE. If there aren't any storms, is that worth it?
If there is bad weather close to Denver, I imagine going south on I25 would be just as bad as going east on I70. I25 climbs to a low pass on the state line, while I70 is downhill all the way to the Mississippi.
Personally I would get information on several routes, and then make a choice based on the 7 day forecasts when I leave. Bad weather across the middle part of the route can come in two forms - storms from the west the cross the Rockies and move out into the plains, or a Canadian cold front moves south and clashes with warm moist Gulf air.