query AD users what syntax?

I have a script that seems to work ,but I am wondering is my syntax ok?  I am only wanting to search user accounts not computer accounts thus i'm using objectcategory.  I'm actually interested in the msNPAllowDialin from a change perspective.  I'm not clear why category person  and class user vs. using objcategory user only.
 
objCommand.CommandText = "SELECT DistinguishedName,samaccountname,msNPAllowDialin,Name FROM 'LDAP://ou=users,ou=acme1,ou=acmeOU,dc=acmedomain,dc=com' WHERE objectCategory='user'" 
 
I have seen suggestions using this syntax.
 
 
"SELECT DistinguishedName,samaccountname,msNPAllowDialin,Name FROM 'LDAP://ou=Sales,dc=MyDomain,dc=com' WHERE objectCategory='person' AND objectClass='user'"
 
 
 
 
 

"objectCategory='person' AND objectClass='user'" is the canonical way to filter for user accounts. "objectCategory='user'" only works because AD is making an effort (section "Other things to be aware of").