i bought pro tools a long time ago and honestly the difference between the mbox 1, mbox2 and i think its called the mbox 2 pro is very insuggnificant. i started with the mbox 1 then traded in to the the mbox 2. the biggest difference is the preamps used for recording that you would plug your mic into. the mbox 1 uses a lowend "focusrite" preamp when the mbox 2 is just some shit outta the closet digidesign amp. however if your looking it at it that way the focusrite preamp involved in the mbox 1 has a minor warmer sound to it and its almost unhearable.
if you considder that digidesign has a tade opperunity every few years it best just to get whats latest. Even still im not how you would acquire an mbox 1 other then second hand and if price is an issue then definatly mgo with the mbox 1 and in anycase when you get some more money you can just turn it in to get a better one.
as for m powered... the m-audio version of protools, its kinda rediculas to go that route and buy a better sound card if 1.) your not a dj or own turntables or any serious components that you would considder recording into protools. 2.) have a budget and are unsure about what your really buying into 3.) like i said before dont really know what your buying into but if you ever want to sell it the mbox is a well known name that pretty much anyone in music will buy for a reasonable price 4.) want other digidesign products like the 002 or 003 in the future. all m audio products are nice but digidesign is definatly reasonable when it comes to delivering a quality product for a quality price.
also a plus if for $around 700 to 800 dollars canadian you can get all that in 1 package(mbox 2 pro)
i hope this helps. i had the same issue and had to do some serious research to find my answers way back when.