I can't see any practical difference between the two formats from the
ISA Management Console, though it looks like the exact field names get
changed slightly between the formats.

Now, whether those minor differences will be enough to prevent log
analysers from parsing the ISA format, I can't tell. Certainly most
will be able to cope with the W3C format.

Personally, I can't see any reason to change from the default W3C
format...