Are doing a raw-edge applique? If so, I'd go with a satin stitch, or else my experience has been that you need to treat the edges with Fray Check or little bits will come unraveled. If you are turning the edges under before you fuse it to your fabric, either one would look nice. Zig-Zag would be fun for a crazy quilt or maybe a really fun child's quilt. A straight stitch seems to me like it would be a cleaner, more elegant look.