I'd always recommend an apartment for a small startup with little money. Even if you don't live in it, I'd recommend getting an apartment as an office. Apartments are cheaper and nicer to work in. The only advantages of offices are that the can hold more people (not an issue for a young startup) and that they seem more professional. But having a good product is more important than seeming professional, and you're more likely to get one if you work in a good place.