I just became aware of the blue magic sealer the other day when I was going through a box of stuff I got from my uncle. In it was some blue magic headlight restoring compound. I quickly read the directions and it said 'for best results, use with blue magic sealer'. According to the reviews on amazon it works about 6 months to a year. That pint of urethane cost me $15 plus $5 for the mineral spirits.
Blue Magic headlight sealer is $8 a bottle on amazon right now.
If it actually lasts, I'd probably prefer the urethane just because I'm lazy and willing to pay a little more (and that pint will last me forever). Time will tell.