If you are wanting a full "stealth" approach drop is better. Deny returns a message stating that access was denied, similar to the http error 403, whereas drop just ignores it and the message just gets ignored and no reply is sent back as if the machine does not exist. [:)]e.
doesn't matter if you use DROP or DENY from the aspect of being scanned. Because both do the same but DENY drops the packets as well and logs in addition to the related kernel ogfile.