I have 3 different CF flyboxes that I use. They are all the medium size boxes, not the large waterproof one that you are looking at. However, one of them is a 12 row with a 12 row center flip page in it. This box gives you a lot of slots for storing flies, but I don't like it for dry flies or larger streamers or poppers due to the flip page. Using the flip page cuts the space in the box to "stack" the flies in when the box is closed. This can be a real issue with larger flies, or with dry flies, as the wings and hackle can be crushed in the box.
The other two boxes I have are the C&F system boxes that have the interchangeable foam inserts. I really like these boxes for the versitility they offer. I can put the high row count inserts in the box when I am carrying small dries and nymphs, or easily swap that insert out for a streamer type insert to carry the large streamers and poppers.
My recommendation would be to go with the FFSL1 box, and to get the inserts for it that give you the flexibility you want to carry the different types of flies. You can set up one side of the box with an insert for small dries and nymphs, and the other side for larger streamers and poppers.
Just my thoughts....