Well, stainless is a product that is said to not cause any harmful effects. We will see what the 'scientists' say in the next 5-10 years.
Plastic is said to not be safe for hot liquids or at all any more. Go figure since everything we have is plastic!
Aluminum bottles typically contain plastic linings on the inside of the aluminum, so they are not safe from plastic woes and should not be used with hot liquids.
We got rid of our BPA-riddled Nalgene bottles (recycled), and then found out about the Aluminum-plastic issue and we want to take hot liquids. Also, we have a case Nalgene Cool Stuff Neoprene Carrier 32 oz for the outside of the Nalgene from the plastic-bottle days, so that's nice. Also the tapered edge means that this bottle fits in our Honda Fit or our Toyota 4Runner. Yay! Additionally, we can use the sippy tops that we used with the original Nalgenes Guyot Designs SplashGuard-Universal
Great product. Exactly what we wanted. So we got 2!