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Panther: Exposé, Safari, Mail, Preview, XCode, fast user switching, X11 plus much more (see http://www.apple.com/macosx/newfeatures/ for all new features)
Jaguar: Networking (IMO), some people also prefer Jaguar's Finder

Hopefully 10.3.3 will improve Panther's networking, and you can turn off the sidebar and/or toolbar in Finder windows. Overall, Panther is definately better, but whether it's worth the cost is debatable.
 
Of course Panther is better, but it also eats more RAM. Run Panther if you have a decent amount of RAM (384MB and up) and Jaguar if you have around 128MB.

I noticed this because Jaguar would stay very snappy all the time under 128MB RAM on my 400mhz iMac. Panther is a whole nother story - I'm running it now, and it gets pretty sluggish sometimes. And to those reinstall freaks out there, this has remained true for several fresh installations.
 
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