If baling straw was extremely important, I would look for a machine with a beater and a standard rotor. The standard rotor doesn't tear up the straw as bad as a specialty rotor. Keep the concaves as open far as you can and still get a good sample, and run smooth slotted grates.
Humidity makes a big difference. If it's 100 degrees and 9% humidity, you're not going to have good straw no matter what.