what is deference between reading session id, from cookie or request?

when you use session in  cookieless="false" mode in web config  it store session id in a cookie value, now i want to know if by any reasons session id change,this cookie's value will update automatically? and what is deference between reading user session id by these two syntax?     System.Web.HttpContext.Current.Session.SessionID       vs     Request.Cookies["Aspnet_sessionId"].Value 
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