Preferable method? Hmmm. I imagine that depends a lot upon your individual church and what people want and expect from the program. I can tell you what we do.
We encourage teachers to come up with ideas and adjust the lesson plan, but most of the teachers (except the teachers on our curriculum team) don't seem to want that much freedom/responsibility. Part of the reason WoRM appealed to them was that it meant less preparation time, not more. Also, we have a lot of people involved that are new to teaching and they want guidance. So we give teachers a complete lesson plan. (That's our church - it might not be, yours.)
Here's our process. We have a six year scope and sequence in place. (Personally, I think that's key) Our process is to have a team get together quarterly -- preferably about six months before the lesson is needed -- and brainstorm what we could do in each workshop. Everyone is asked to look over the scripture ahead of time, and make notes. We tend to brainstorm four six week sessions at a time (we do Wednesday nights, too) which takes about 2-1/2 hours.
Part of my job is to have an idea of available resources in video and computer to help us make those decisions. I also usually scan this website and jot down a few ideas. As a group we decide on the lessons and the "angle" we will use in each lesson. Then one person (again, me-the paid staff) writes it up into a tidy lesson plan complete with Biblical background, list of supplies, goals for the session, memory verse, and opening, main, and closing activities. We also include suggestions for if you have too much or too little time, adjustments for age levels, etc.
I take 2-3 months to write up the lesson plans and hand them back to the curriculum team for input. I make changes, and locate teachers. We want to be sure that format is consistent and that everything is in one place when we go to reuse it at the end of our six year rotation. More work now, means less work later.
Regardless of whether one person or many people write the lesson plans, you should probably have a consistant format that you use. The Writing Team Lessons posted on this website are good formats to copy. If your teachers do want to design their own lessons, encourage them to turn in some notes so that 3, 5 or 6 years from now you aren't "reinventing the wheel."
Hope this helps!