Previous Droid 3 owner here.
The size difference isn't really all that telling when typing on screen.
I suggest you install Swiftkey 3 onto your Droid 3 and use it as the only keyboard for a number of days before deciding if you want to move away from a dedicated keyboard.
Personally, I found myself slowly gravitating towards on screen typing the last 5 or so months with the Droid 3. As a result, onscreen typing was faster and more accurate than keyboard typing towards the end. Mostly because I wasn't pushing the physical keys hard enough all the time.
I have no doubt that anyone can become proficient with an onscreen keyboard. But you should take a few days (at least) to decide if you *want* to become proficient with one.
One other thing I will say, the Droid 3 now feels like an old Sovet-era tank compared to the light and nimble GSIII...
-Suntan