Onyx has done a nice job updating the Boox A60 over the last year or so its been on the market. Its a much better device than it was originally and even at launch the software was better than most.
If I were you i would pick the one that is easiest for me to get and try it for a week or two. That probably means the 903 for you. But if you will definitely be using the browser on a regular basis I would say the M90. The browser is just much better.
edit- we dont stock the 903 yet and looking at orders right now I believe we are out of M90s until the next shipment arrives