I am preferable to using 2/0 Flying Dutchman New Spiral blades, when I opt for a spiral. I will cut stock up to 1/4" thick with them, but I wouldn't recommend that. For 1/8" they work great. Like Jim said though, material matters as well. I don't know how they perform if you are cutting ply. I usually cut my detailed fret work in maple, and these blades cut maple like a dream.
Whatever you choose, be sure to try them on a few scrap cuts before you dive into a portrait. They take a little getting used to. And same goes for when you switch back to a straight blade. The first time a used a spiral I liked it, so I did a few more projects and got really comfortable with it. Then I went back to a straight blade for another project and about ruined it, but it doesn't take long to get a handle on it.