Both have pros and cons, but here are the facts. Both aluminum and steel bottles are recyclable. However, aluminum takes a great deal of energy/resources to mine, and is not very environmentally friendly. It has also been linked to neurological disorders and Alzheimer's (with aluminum contamination in the body starting as early as birth considering most vaccinations contain it). Out of the popular bottles out there I would recommend Klean Kanteen over Sigg based solely on the material.
Here are some of my picks for stainless steel bottles and a sippy cup: