Qualify or Keepalive - which is better to use

;keepalive=60 ; Interval at which keepalive packets should be sent to a peer
; Valid options are yes (60 seconds), no, or the number of seconds.
; Default: 0

qualify ; Helps with NAT session
qualifyfreq=60 ; Qualification: How often to check for the
; host to be up in seconds
; Set to low value if you use low timeout for
; NAT of UDP sessions

Phones are NATed behind a Cisco Router and goes through a PIX with SIP fix-up. Its a very small site with 10 phones. We plan to set a very high UDP nat timeout value at the site router and less frequency keepalive from Asterisk to keep the chatter down on a very low bandwidth link. We ran some number calculations and the site router should be more than capable to have the additional nat translations.