303 vs. 304 stainless steel

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.

304 likes to have dull bits for lunch. It grabs them and twists them off mercilessly. My limited experience with it is that it's better to use something else (anything!) instead. It does look nice though polished up.
For things like guards 416 is better than the 300 series for machining.It's a free machining grade of 410 and can be HT'd
There truly is an amazing difference betweem 303 and 304 when it comes to machining.
Like already said, 304 is so gummy it just grabs end mills and drill bits and taps and breaks them left and right.
303 is much more machine-friendly, harder, more scratch resistant and available.
I've used LOTS of both 416 and 303 and can find little difference when it comes to making knives.
(I think guys like the 416 simply because 4 is bigger than 3!)
Nickel makes things 'gummy' .303 has it but 416 doesn't. Anyway they both have Se or S for free machining.