I haven't used either, yet, but the popular opinion seems to be for the Lact-Aid. The Lact-Aid makes the baby work for the milk, as they should at the breast, where an SNS works on gravity, so the baby doesn't really have to work. If you decide to order a Lact-Aid, those with experience have recommended that you order up to six, if you can afford to. At least two or three, though. This way, you don't have to clean it every time you feed, and you can have some bags pre-made in the fridge, so your baby doesn't have to wait as long for you to get situated. What you can order is the Deluxe system, at Lact-Aid, and then you can order individual units, which are about $18 each.
Good Luck!