I'll start with sisal - if it gives us any trouble I'll try poly next time.
the sisal 9000 we use in a 24t works well enough in that baler that we've never tried anything else.
I guess it's like somebody here said, whatever is used, the trick is simply to keep things adjusted properly.
And in reality, with the 4h kids this baler's not going to see a whole lot of use - probably only 150-200 bales a year across two cuttings at the most.
So at those numbers, even if it's not perfect, rebaling a few isn't a big deal.
I'll repost our results when we test it out.