Is it DMAIC or DFSS for online companies?

First, some definitions...

DMAIC - Define, Measure, Analyze, Improve, Control -- the 5 phases of Six Sigma

DFSS - Design for Six Sigma

DMAIC - deals with variation/defects in existing processes

DFSS - deals with designing new processes/products without defects correctly the first time

Employing DMAIC at an internet-based company, reading between the lines, means that the product offering you are serving up to customers, has errors; plain and simple. Those errors could be related to design, customer experience, the support offered post-purchase; and many other features and functionality that comprise most eRetailers and other Internet companies.
Looking at DFSS, this seems like a better option since it deals with the design aspect up front - imagine a world where your computer doesn't crash or you don't have to find your installer files in order to make your Windows software or other OS work -- Seems like a fantasy world, right?
Well, the whole goal of DFSS is to ship products defect-free, even at the expense of the launch date slipping (usually, associated with an opportunity cost). What most people forget or perhaps believe, is that the opportunity costs are usually diminished by the impact to the bottom line because of the incurred ops and labor costs incurred to fix something already shipped, in terms of calls to the call center, help desk technicians on a 24/7 schedule can be costly, and if you outsource your phone volume, there is the whole argument that US customers want to talk to US agents. OK - so you say "we will use email and self-service online only." Speaking from experience, unless you are Amazon, who are the only online presensce to successfully service their customers online, thus, prompting them to bury their (800) support # . But for everyone else, customers want the comfort when they make a purchase, of a customer support numer, especially considering that customers are still not fully comfortable entering their credit cards online to purchase. Paypal and phone-me-back options are becoming much more common, as providers realize customers are growing less comfortable, rather than more comfortable, entering their personal details on the web.
Depending on whether you care about the top or bottom line, one of these two cost risks, will rise above and become your priority.

In a start-up, usually, it is the top line growth; sacrifice service costs for the revenue growth opportunities; which I would agree with. And, neither DMAIC or DFSS should be employed.
But, for more stable, operating corporations online, DFSS, becomes a key to your success, but only after you launch a successful DMAIC program with black belts and green belts infused throughout your organization hierarchy.