O.K. Yes, you can record MP3 directly to USB, sort of. You actually record the song and it prompts you to save it. You can save it to internal or to USB as you choose.
The vocal harmonizer works quite well, and although I liked the Harmony M and had no trouble with it (after update to new OS), the one in the Pa900 is better.
It's nothing earth-shattering, just a good solid harmonizer, easy to access and adjust. It's easy to customize. You can adjust just about any parameter you can think of including e.q. pan, choice of voices, gender, etc. WAY better than Yamaha's for sure.
There seems to be a lot more headroom to play with the mic input on the PA3X has.
I have not played or even seen a PA600 so can't help much there.
If Uncle Dave likes the harmonizer it must be pretty good!