As big a fan as I am of Argento, I was never all that big on Deep Red. The effeminate "hero" just didn't work for me. I did enjoy Daria Nicolodi's performance, though.
Suspiria, on the other hand, is a delight in pretty much every way. It was the first Argento flick I ever saw, at age 16 or so, and it blew me away. The score, the visuals, the sets and the emphasis on primary colors all elevated it to a sort of hyper-reality you don't see in other films, and when you do, it's never quite as effective. It's not the best acted movie and, as is de rigeur for Argento, very little of it makes any sense, but Argento's artistry more than makes up for it.