Allison,
Thanks for your incredible patience in hearing back from me!

I'm immersed in writing, writing, writing right now and can only get on the board sporadically.
In thinking through what you've shared, I am leaning toward your older son doing The Exciting World of Creative Writing as scheduled within Rev2Rev.

I think it will be a good change from the IEW he's done the last two years. There is so much training in The Exciting World... on writing to God's glory. In my opinion it does this aspect better than anything else I've seen.

It also takes kiddos completely through the writing process in a less formulistic way.

It is full of outstanding examples of good writing from classic literature as well as from Christian authors. This modeling from excellent passages makes the lessons so interesting and very CM!
As far as your second son in line goes, I'm thinking that if he has never done much in the area of descriptive writing, it would be good to go through WWTB with him. Kiddos don't need to have WWTB Vol. I in order to be able to do Vol. II, but I think that the jump from IEW's All Creatures Great and Small up to The Exciting World of Creative Writing would be fairly large. So, I think that WWTB would be a good stepping-stone for what will come next. You'll know better than me, but I also have found it's good for my older two boys to be in different writing programs if possible so there isn't so much comparison between them in the writing area. This is why my oldest did Medieval History-Based Writing Lessons on a different year than my youngest.
Anyway, those are just my thoughts!

If you do end up with WWTB for your younger son, I'd alternate it with Rod and Staff grammar, so it doesn't get too heavy for him. If you happen to have the CTC guide, I'd just follow the schedule in that language arts box for your younger son for the year to get a good balance of dictation, DITHR, grammar, and writing.
Blessings,
Carrie