Maybe it's an advantage if the competition thinks you're crappy, when in reality you're building exactly what their customers want.
But that's not enough to win this argument -- I assume you've already talked to him about "4 steps" and that it hasn't worked. More arguing is probably not the answer.
Sometimes you do know the market cold, in which case you do know what to build. An example is an entrepreneur who just finished a 5-year stint at a similar company and saw all the opportunity and all the mistakes. Another example is someone who has detailed mock-ups which were shopped around and "proven."
I also agree with others above that perception does matter in certain industries. Imagine a design and marketing firm with a crappy website!
However if both of you are stumbling around in new territory, I think the main thing to challenge is the notion that either of you knows what "full featured" means. If it means "Copying a lot of the features of competitors," then why are you in business? If it means "A unique approach," that's better, but then you have to prove the concept along the way.
After all, surely there are 10 ways you could do it, all rational, and most won't turn into a business. That's not good dice to roll!