The difference is that in NitDroid not everything you can do on android is supported yet, so some functions do not work. It is a work in progress, but it is usable. I suggest you get a bit more comfortable with the Maemo5 system before digging into the NitDroid thing. See the wiki pages under the cummunity tab of the site for detailed instructions. There are a lot of topics about that already, there is a search button in every thread as well. Remember to backup your data preferably to SD, and backupmenu is a great tool as well. Nice to nee new maemo members by the way!

I was once new to Maemo5 and managed to do all kinds of cool stuff too including NitDroid dual booting and UI modding.

Good luck