We have never used MFW, but we have been using FIAR for 2 years and love, love, love it!!! As for prep work with FIAR, you can do as little or as much as you want. Some people do FIAR conversationally, while others (like us) lapbook/notebook each book. FIAR can be very low cost, if you can get the books from your library. Since FIAR is unit studies, you can use FIAR for many years just going deeper with the subjects each time you "row" a book. You would need to add a phonics and math program to FIAR.
If you are considering FIAR, look at http://www.homeschoolshare.com/ all the titles are listed with lapbook/notebook ideas you can use.
My blog will give you an idea of how we use FIAR
HTH!