The SPWeb.SiteUsers method run the following stored procedures in SQL:
proc_GetTpWebMetaDataAndListMetaData
proc_SecListAllSiteMembers <dbo.Sites.Id>
The SPWeb.AllUsers method run the following stored procedures in SQL:
proc_GetTpWebMetaDataAndListMetaData
proc_SecListAllWebMembers <dbo.Sites.Id> , <dbo.Webs.Id>