As it was explained to me, the main difference is that you can have $lots of prefixes in IS-IS without it falling over, whereas Dijkstra is far more resource-intensive and as such OSPF doesn't get too happy after $a_lot_less prefixes. Those numbers can be debated as you like, but I think if you were to redist bgp ospf on a lab machine you'd get the point.
Disclaimer: I've never run IS-IS operationally, just in the lab. -Jack > Which makes no sense to me. I originally looked at both and thought OSPF to > be inferior to IS-IS. That being said, OSPF is supported on more (and > cheaper) hardware. IS-IS can have additional licensing with some hardware > (where OSPF does not) and is often considered a "service provider" protocol > by vendors. > > > Jack >