Prototyping vs. Clean Code at the early stages

Question

I'm planning to work/start on a few personal projects that could end up as my daily job. It made me think, which way should I start?

• Just prototype—write just working basic code that could cost me tons of time optimizing and refactoring for easy expansion.

• Write clean, optimized and documented code from the very beginning, keeping in mind that if after some time it won't be cost-effective, it will be dropped.

Update: Combining YAGNI with sunpech and M.Sameer answers makes perfect sense to me :) thank you everyone for help.