In a nutshell - Bike Week is bigger but the weather is usually better for Biketoberfest.
We completely agree. Bike Week is often cold and rainy (from my previous 3 years of experience). However, H-D is usually there for demo rides in the spring, but not in October. I can attest that the weather is great for Biketoberfest (as I type this response from my Daytona balcony during Biketoberfest
Either way, you'll really enjoy it.