Most likely the two stainless steel alloys (304 and 316) mentioned in your link will be non or only weakly magnetic. They are both austenitic stainless steels, which are only occasionally magnetic, mostly after cold working.

I'd steer you towards either the galvanized plain carbon steel or the black powdercoated perforated steel as a better option for a magnet board, if you require the perforation. A nice chromed steel would be pretty, but I don't see it offered at the site you linked to. Ferritic (note not ferrous, ferrous only means iron is involved, and there's iron in all steels, stainless or not) stainless steels are magnetic.

Some good background on stainless can be found here.

Best,
-Zeke