I saw both Susan Spencer and Kate Fisher, and I enjoyed them both. It's kind of hard to compare them because I felt they did very different things with the character of Jo (which is kind of amazing as I got the sense the show is pretty regimented). Fisher was what I sort of expected to see from a Jo, and that's certainly not a bad thing; she really had the tomboyish charm going on. Spencer seemed to me a little more delicate and maybe too pretty (vocally and physically), but she was still spunky and came across well.
And of course there's the added sweet touch with the Jo/Professor relationship with Andrew Varela and Susan Spencer, as they're married and adorable together.
'"Contrairiwise," continued Tweedledee, "if it was so, it might be; and if it were so, it would be; but as it isn't, it ain't. That's logic."'
~Lewis Carroll