I bought the ridgid R7121 for mixing drywall mud and thinset/grout for tiling. It was between this and the Dewalt DW130V, i chose the Ridgid for the life time warranty but I now realized it's a single speed drill (pull and go). The Dewalt is a variable speed. The variable speed sounds more practical for those materials. Does anyone have any opions??? Or preferred experience w/ either???