Depends on how the routing scripts are designed and also what database fields you are using for ASA. For a Call Type, if you are using AnswerWaitTimeHalf / CallsAnsweredHalf then ASA will start calculating from when the call first enters the Call Type so if for example, your routing script sets the Call Type, plays a couple of messages, does some expected wait time checks etc before queuing the call to the skill group, the ASA for the Call Type will also include this time where a Skill Group will only start counting from when the call hits the queue to skill group node so the ASA on the Call Type in this instance will generally be higher than the ASA for the Skill Group.

Depends on what you want to measure. Call Types measure the caller experience (ASA on a Call Type is the actual average delay a caller waited before speaking to someone which will include things like messages played to the caller while in the call type etc) where Skill Groups generally measure agent in a skill group's performance (ASA on a Skill Group is how long calls waited in the queue on average before being answered by an agent).

Cheers,
Nathan