ntpdate is deprecated as of September 2012; apparently ntpd now has the ability to do one-time updates if needed, and ntpdate is based on "long-neglected" ntpd code. (News to me, since my system has ntpdate but not ntpd! I'll be fixing that presently; thanks for asking this question.)

As for the difference between continuous versus periodic updates, I think the trade-offs there are what you would expect; continuous adjustments maintain better accuracy over time, which can be important for databases and MailDir (see same link). But of course this would require slightly more network traffic (presumably negligible for exchanging simple timestamps) as well as an always-online network connection, slight consumption of RAM and CPU, etc. But all these downsides are tiny in a modern computing environment; overall I doubt there are good reasons to substitute cron for ntpd.