The LactAid is preferred by most on this board, although I am sure not by all. The LactAid has a more flexible tube then the SNS. The SNS has a hard plastic bottle, which is harder to hide in your clothing when nursing in public. The SNS lets the milk out due to gravity, where the LactAid makes the baby work for it, just as they normally do at the breast. But I think the SNS has 3 different tube sizes, where the LactAid has just the one size.
I don't have any experience with either one of them, although I do have 3 LactAids on hand, just in case I ever need them.
Hope this helps!