どうぞおかけください is more polite than どうぞ座ってください。

I'm not a native speaker though.

All right. Some native speakers, especially some middle-aged people, choose a slightly less polite way of saying to be friendly, but it is difficult to do it well.　I never recommend this way. I think it would be better to use polite expressions with 尊敬語 or 謙譲語, and to try to show a friendly attitude with your smile and your tone of voice. A lot of people think this way: 親しき仲にも礼儀あり. So, I don't think politeness hurts friendly atmosphere at all especially between you and your clients.

Misaki is right. Plus, I would basically avoid saying どうぞ座ってください to a client because it might sound as if I think (or want to show) they are equal to or inferior to me.

Thank you , Misaki and Haruka, now I know how to ask my Japanese clients to have a sit. And I guess it's really improtant to know which one is more polite if you wanna speak a native language of your clients to show your friendly.