Is a 2013 Dodge Charger with 70k miles better than a 2015 with 100k miles?

If they are retired police vehicles then they are both suspect. Retired cop cars can contain all sorts of hidden surprises. I used to buy retired patrol cars from police auctions, but stopped when a lot of them began retiring the Chargers. The Chargers were not holding up at all well to the abuse, compared to the discontinued CVPIs. Regarding your two choices, I would prefer the lower mileage car, but it really depends on how well maintained they were and what sort of abuse they took. If both are coming from the same location, then assume they had the same level of maintenance.

Both have hundreds if not thousands of hours idling in addition to the miles. I do not know if they also have hour meters but if they do see what the hours are. Don't buy them unless they are super cheap.

I hour of idling equals 33 miles.