"Both have the same idea- break down balls in play by type, location, velocity."
How is it measuring velocity? I know that a human determines where the ball lands. (Am I wrong? I suppose I could be. I'd put a little more stock into this if we had computer tracking the ball.)
"Defensive metrics are less reliable than offensive metrics"
By reliable, do we mean accurate? If a batter bats .300, I know he has 3 hits every 10 atbats, and can make an assumption on how productive he was to his team. If a player has a UZR of 10, can I assume he saved his team 10 runs to the average fielder? I'm not sure you can say yes. I can say with some confidence that Trout did not save his team 3 runs over the past few games, compared to the average center fielder. If you believe he has, share with me why you feel this way.
Over the past 4 games against the Yankees, Trout went 5 for 14, walked 3 times, hit a homer, drove in 5 runs, got hit by a pitch, and stole a base. I can look at these numbers and tell you he was very productive based on the results of his actions playing offense. Do you feel confident that you can say the same re: his defense? If no, then it would be unfair to use UZR as a means to tell me how productive he was defensively. And for statnerds who go "BUT WARRRR" when it comes to arguments of who the best player is, you should reconsider.