Hi Chris,
The client-side sorting script was one of the first I used, and while good, it's proven to be slightly less flexible than I'd hoped for.
To cut a fairly long story short, the client side script "kicks in" when it sees a table with the sortable class. So client-side is fixed to "sortable". It's on my list of things to re-visit, but in the meantime I'd recommend you stick with the server-side script.
Incidentally, there are other issues with the client-side script - most notably that it doesn't "remember" the sort order you last chose.
Cheers
Bruce