I've seen the difference in datum as well. In the case where we found it- .10 mile in the area of a group hike.

From the air that's not a big deal. By foot in the woods - it matters.

I like UTM because the grid lines are frequent enough on the map you can find where you are. USGS quads don't have frequent enough lines to suit me. And then you are stuck interpolating between intervals of 2 minutes 30 seconds per line.

I've written about this on my website-
http://coloradowreckchasing.com/UTMesHiking/UTMeshiking.html