I would go for the 570 for the extra harp space. Also, I assume the 440's are second hand? I would prefer to buy new with any computerized machine, as it's the computer boards that will go bad eventually, and eventually they become hard to find if you need a replacement.
I've also learned my lesson about skimping on harp space. In the long run you save money by buying the larger machine first, rather than settling for "good enough" and then realizing it doesn't actually meet your needs.
Whichever one you choose, I'm sure you will enjoy it!