When you decided to switch to new Blogger, your template won't be changed, so you will have an option to switch to new layout (using widgets) or stay using the old template. I have been trying to migrate to the new layout, but it's too much efforts, since i have to add a lot of elements on it to make sure that everything in my old template exist on my new template. Besides that, i have made several changes to the HTML code directly, and some of them cannot be applied to the new layout, even though i have changed the code directly, but still failed. So finally i decided to revert it back to my old template (luckily Blogger saved my old template before switching to the new one).
Even though i won't be able to enjoy the drag-and-drop features using widgets in the new layout, i prefer the old template since the Fat Tux is still there (it's one of the code that didn't work in the new template) :)