Pretty much night and day.
304 is tougher and "gummier", for want of a better phrase. It likes sharp tools and is a friggin' bear to tap. Unless you absolutely need the properties of 304, stay away from it.
303 is by far easier to drill, tap and turn. Like any stainless, it'll still work-harden if you let the cutting tool dwell, but even here it's more forgiving than 304.
Doc.