After evaluating both platforms in more detail I will probably start with freelancer.com as it is possible to have an arbitrary name (e.g. "elephantdesign") instead of real name only. This makes it simpler to tailor your profile to a specific niche in my opinion and seems more privacy aware. Obviously both platforms are pretty intrusive, but at least freelancer.com guard your real name.

However there seems to be an agency mode on Upworks which might be similar.