Stealing a police car is usually a much more serious offense than stealing someone else's regular car.
If you steal someone else's car that is usually considered a misdemeanor if it's your first offense and might get you 1 year in jail and a fine.
Now if you steal a police vehicle and then take the police on a police pursuit then you'll likely get a felony charge on you plus the fleeing and eluding from police especially if you don't immediately pull over.
Stealing a police vehicle or police car is usually punishable with prison time of 2 to 15 years and if you did something else more serious and it's your second or third offense then you might be looking at serving 20 to 30 years in prison or even possible the rest of your life in prison.
It's never a good idea to steal any car let alone a police vehicle because you'll be in either jail for awhile or prison for a long time as a result.