When you are a startup with no money, I would say buy the .net first only if you could foresee buying the .com in the future when you do have the money. Example of this would be if the .net was available and the .com domain was owned by a squatter and is willing to sell it for $2k to $5k.

I wouldn't by a .net domain where the .com is already owned by an established company because you probably won't be able to buy it in the future.