Writeily Pro is an awesome Android app, and open source. It can save its files to any directory on your SD card, so as long as you get that directory syncing, you're good to go. ie. it's sync service agnostic
nvAlt never played well with Dropbox for me. I tried to use it at home and at work, sharing my notes/documents via Dropbox and I had all kinds of sync issues unless I was super dliligent about closing the client on one machine before opening it on the other (something I didn't always remember to do and once I'm at work, I can't close the home one and then can't use it all day etc)
