Depends on how big your products are, how big your softbox is, and which one you can get at a better quality for a better price. I'm sure either will work. You've got a 14-45mm lens, take some test shots at 35mm and 45mm and see if you like one better than the other.
It depends on what you'll be photographing, but in general, you don't want to have distorted product images. I would get a 50mm, and even a longer one like an 85, to compress the image and minimize these distortions.