White Spruce seedlings/seeds
White spruce get huge remember. I would personally go with the seedlings. I think they are easier to manage, if you are going with a formal look. At maturity (sure why plan for 50 years from now...) the white spruce will only need to be 20-30 feet apart and still will have thier branches reach the next tree, of course you will want to thin out the line as it grows, so some mortality is to be expected. I don't think that price is much of an issue for you really what's 20 bucks difference in one purchase. If you were to keep making purchases like this for a company over and over, then seeds might be more efficient. Also if you want the trees planted informally, or more naturally seeds might be more effective, you will have more trees, more randomness. If there is already vegetation present the seedlings will have a distinct advantage over seeds.