A stripe bead is just that- a bead that is striped, but a porphyr bead is one that had two or more striped or swirled patterns.
Generally, porphyr beads are swirled with white and another color; sometimes it may include clear, in addition to white and another color. Stripe beads can be multiple colors; you may even see stripe beads that have a multiple background colors and a different color stripe.
Most porphyr beads end up with more of a swirl pattern than a stripe. A stripe bead is generally made of even, parallel stripes of any color; sometimes the lines may be curved or squiggled, but they generally remain even.