I just ran my new Digitech which fits nicely on my keyboards through its paces. it kind of works along the same lines as TC's Harmony M which I found to be extremely unreliable. I have two harmony voices to accompany my lead vocal and they are set up by pressing a button to go to High, Higher Low, Lower, Double etc. I can also do gender shifting. I am able to store five settings and it reads my keyboard chords perfectly. So I really don't need the 900 for my harmony needs unless it has some great settings that you wouldn't be without.
If the 900 can record mp3's directly to a USB drive that would be a feature well worth having that is lacking on the 600.
Many of the styles I used for my original songs when I had a Pa 800 rented for a month are on the 600 and that is a plus. I'm definitely trying to choose between them and mostly wondering if the 900 would be worth the extra $1000. I would have to start saving to acquire the extra thousand.