They will be the same. However, they don't use measurements as size like manufactures do with jeans. Here is a resource to figure out which size you need:
As long as you order from the same manufacturer, they should be virtually the same. However, I recommend you buy surplus since you can get so much more for so much less money. Plus you get some sweet novelties like the US flag and division patches, but I HIGHLY recommend you remove the name and rank patches. This way, you'll also be certain that you'll be buying a reputable brand that's durable.
Yeah, I quit airsoft. No regrets.