What's harder: running a 5 minute mile or cycling at 5 W/kg for 20 minutes?

Cycling at 5 w/kg for 20 minutes is much harder.

I ride 4-5 hours/week as cross training (I used to do a lot more) and my best 20 minute effort is 4.5 w/kg.

I can run a 5 minute mile 3 times in a row though.

Watts per kilo isn't the best benchmark because it's a lot easier for lighter riders. However, 5w/kg is way harder at any weight compared to a 5 minute mile. Running a sub 5 mile with minimal or no training isn't unheard of, but there are very few people who can ride at 5w/kg for 20 minutes with no training.

5 Watt per kg for 20 is a lot harder.

Most people who run under 5 simply happen to have efficient running biomechanics. You need to be strong and fit for 5 w/kg for an extended duration.