Rendon is currently 3 wins better than Murphy 6.5 vs. 3.5 (fWAR). That is a significant difference. A lot of this difference is due to defensive ability, but not all of it. Rendon has a higher wRC+, which means more runs created (144 vs 133, 100 is average), a higher OPS (.948 vs .922), and higher isolated power (.240 vs .225). His BB/K ratio is higher than Murphy's (0.96 vs. 0.67). Baserunning (BsR, which is a stat that combines stolen bases, CS, taking extra bases, etc.) for Rendon is 3.4 vs -1.1 for Murphy.
Murphy has a higher BA (.318 vs .304).
Rendon gets on base more, has better plate discipline, hits slightly better for power and creates more runs and is a better baserunner and a much better fielder.
Those three wins represent a huge amount of difference.