Both of them are good keyword research tool. MNF basically for newbie. you input keyword and it will tell you should you go for that keyword or not.

on the other hand MS good for advance keyword research. it's not only do keyword research but also have some other module such as competition analysis.

i have both but rarely use MNF. i use MS for all of my keywords research task.

last but not least if you thinking to buy...i will advice you go for MS. it will be best bang for your buck.

MS is also noob friendly!

there are great tutorials on the website - I had no idea how to do KWR and now I'm pretty decent at it (really looked at the videos a week or so ago) picked it up really quick!

The main function for both tools are same, if you're newbie you can use MNF, since its more easy to use and understand. But if your are advance, you can go for MS. And I use MS for long time, and have good result with this tools.