older or younger?

Hello, I'm not yet a mother, but hope to be both a bio and adoptive mother one day. My husband does also, and we are are trying to put a lot of thought into planning our family. We think we'd like to have two kids, but perhaps more. Does anyone have any opinions on what would be best emotionally for an adopted and bio siblings--should the adopted child be the youngest or would older be good too? Any thoughts on age differences, best ages to adjust, etc? Should we stick with even ratios, or perhaps try to adopt a sibling pair? Thanks for any advice!