What sells better simple design or fancy design?

[SykkBoy]

I was consulting with a company who's members areas are the typical generic niche content, a lot of it, but not much in the way of updates and their retention rates weren't that bad...their tours were fairly plain jane and yet their conversions were still good (mainly opt-in mailer traffic)

so, I told them to keep the fancy designs to impress affiliates to send traffic and then use the "regular" designs for their own traffic, mailers and SE's...