I have been asked which one - Sonlight or FIAR? This is a very tough question because so far I have not limited myself to one but we are using both so here goes ...
We first started looking for a homeschooling curriculum in 2005 and bought both Sonlight and FIAR at the same time. One came in a box with everything I needed to teach every subject (except Maths as we choose to use Maths-U-See and had to buy that elsewhere), the other came from everywhere I had to hunt and buy the books as I found them, even now I do not have all the books as some are out of print.
Both contain beautiful stories to read and share with your children. Many have become old favourites. I cannot read the PK or B4 FIAR books to just my littlies without the older children stopping what they are doing and wanting to join in listening.
We chose Sonlight because it contained everything I don't have to hunt for the books, or skip the ones that are out of print. The IG (Instructors Guide) schedules all the books for me and gives extra activities and things to talk about as we go. CONVENIENCE. When I first started homeschooling I wanted a guide so I could just tick the boxes and not have to worry if I was covering everything. But I wanted something that involved lots of reading and a very gentle way of learning in order to reflect a Charlotte Mason style of homeschooling. Sonlight has been this.
We choose FIAR because I love the FIAR stories and how you get to spent time slowly reading the books. A lot of the same books are used in Sonlight and FIAR. The difference I have noticed is that Sonlight introduces the books earlier for example The Boxcar children is read in K (5-7yrs) but in FIAR it is read in Beyond FIAR (8-12yrs).
We tend to go slower through Sonlight and add in the FIAR stories to fill in the gaps. Currently a Sonlight core takes us about 1 1/2 years to complete because we lapbook and notebook and add in extras as we go, plus babies and lots of moving house in the past 2 years has slowed us down. We are starting to speed up as the kids grow and life changes so I don't expect the stretched out cores to continue. Although we will still continue to add in extra things as we go.
I like how all the girls (ages 4-9) can be reading the History and Geography Read alouds together but be working on Language Arts at their own level.
Sonlight is focused on reading books and so you read them faster than you do in FIAR. For example you read the story about Ping once but following FIAR you read it everyday for a week while you focus on a different aspect of the book daily. Some kids will love this, others will hate it.
FIAR is not a complete curriculum. You still need to plan out the lessons although the activities are in the manual, you also need to add in a language arts program and a maths program from somewhere else. If you like planning things for yourself then FIAR is an option. Homeschool Share has lots of extra things including lapbooks to go along with the FIAR manuals.
It really depends what you want, personally I think Sonlight provides a better coverage as you can start at age 3 and go right through high school and is planned into time periods of history. FIAR has no real linking between the books chosen in each level other than that they are all good books. They are currently introducing Above and Beyond FIAR which goes past 12 years of age (at this stage there is only one book planned but I am sure there will be others). I personally feel that Sonlight will be a better choice for our family as our children get older and I can see FIAR slowly being put aside and Sonlight becoming our main curriculum.
My only complaint with Sonlight so far is the Language Arts program. I am still struggling to get this to work for Zebra BUT she has had learning difficulties and struggled from the start. But this is the only area of the program where I feel like I am having to "rewrite" it to fit my child's style of learning.
I hope this answers your questions. If not leave a comment and I will try to answer it.