No matter how you slice it Arenado has the better stats according to Baseball Reference:
Home runs last 4 years:
Arenado - 42,41,37,38
Rendon - 5,20,25,24 (21 5 years ago)
RBIs Lat 4 years
Arenado - 130,133,130,110
Rendon - 25,85,100,92 (83 5 years ago)
WAR the last 4 years:
Arenado - 5.8, 6.6, 7.2, 5.6
Rendon - 0.4, 3.9, 6.0, 4.2 (6.6 5 years ago)
Arenado also has a Gold Glove.
No, just No.
If you look at numbers that correct for ballpark, OPS+ and wRC+, Rendon has been better offensively
Rendon wRC+ : 2018 140, 2017 141
Arenado wRC+ : 2018 132, 2017 129
Rendon OPS+ : 2018 137, 2017 139
Arenado OPS+ : 2018 133, 2017 130
If you look at traditional stats that aren't home runs and by extension RBI's, Rendon is equal or better
Rendon BA: 2018 .301, 2017 .308
Arenado BA:2018 .297, 2017 .309
Rendon OBP: 2018 .374, 2017 .403
Arenado OBP: 2018 .374, 2017 .373
Rendon K%: 2018 13.7%, 2017, 13.6%
Arenado K%: 2018 18.1%. 2017 15.6%
I don't necessarily like defensive metrics, so I chose 3 different ones
UZR/150
Rendon: 2018 6.5 2017 12.9
Arenado: 2018 5.2 2017 5.9
DRS
Rendon -6, 7
Arenado 5, 20
Fangraphs Defensive Rating
Rendon 7.9, 14.3
Arenado 8.1, 9.0
Maybe on balance to Arenado, but it's much closer that the talking heads on TV would have you believe. And Gold Gloves are a joke and almost purely reputational. Once you win one, it's so much easier to win another.
Arenado plays in Coors. Maybe he would still be good if he played away from there. But now we will never know. But what we do know is that if you look at any stat that doesn't involve power, Rendon is a better hitter and their defense is similar.