LPU's cost on average 3 times as much as modified alkyds like Brightside. A gallon of Brightside is about $100 retail, while AwlGrip will run $270 to $330 retail depending on color, while other brands, like Sterling can $310 - $350.
Yep, the LPU's are more scratch resistant, harder, have higher gloss retention, etc., but you have to pay for it. Additionally, LPU's are not especially user friendly for the novice
painter. So, if you've got some experience with converters, wetting agents, accelerators and the like, typical of two part linear polyurethanes, then go for the good stuff. If this seems a bit overwhelming, then stick with plain old alkyd paint, where all you need is some spirits and possibly Penatrol to "flow" things out. In all honesty, recent advances in modified alkyds have made them very good. They flow out great, you can roll and tip to nearly a spray like finish, they're harder and more durable then ever and they can be buffed to nearly LPU standards, all at 1/3 the cost of an LPU.