Hi lite is much finer in particle size than pearls. Pearls work thru the entire mass of plastic while, even though throughout the entire mass too, hi lites work on the surface of the plastic. Efishnc's link showing the purple baits does a super job of showing how each works.
Some interesting effects can be gotten using the hi lite and clear together in clear or near clear plastic....pro blue is one such a color. Pro blue has blue hi lite in a thin smoke color over a thin pearl.
Hi lite colors are intended for use in plastic but some super reflective color twists can be had by brushing pure hi lite powder straight out of the jar ON a baits surface and then cover dipping in clear. This process seems to be most effective when the hi lite is brushed on darker colors but will work on almost any color. You can create some super minnow imitations doing this so I'd urge you to try it. The bait shown here is based primarily on a near clear plastic loaded with a pearl called super sparkle with the chartreuse color line thru it, adding eyes, then finished by brushing a blend of blue and green hi lite powder across the back then cover dipping in clear.

Here's yet another brushed number...

And here's some hi lite done IN the plastic to show the difference between surface application and in the plastic.

Finally Les, here's an example of the simple hi lite powder Do-It sells used in an air brush clear as a top coat to enhance the air brushed colors underneath. Look at the tool marks in the back of thee baits and see the "tiger" striping. The hi lite is so fine that it literally puddles in the clear coat where there's an indent to create that striped effect.

In my book pearl go into the plastic and that's the end game for pearls. The hi lites? Oh no....they have a far broader reach in what you can do with them and they have a whole fascinating world to explore using them and I'd urge you to dive it.