The HI-Power Silent is a full 'shine' grade stainless, 304. The HI-Power 409 is made from an exhaust grade stainless, 409. so not shiny, the cannon at the end is, but not the pipes. the HI-Power Silent usually retails for around $100 extra as well.
i have a HI-Power 409 in my stagea and am very happy with it. very good quality, no drama installing it, eveything lined up perfectly.