The standard is .030 and the LW is .025. It's not as bad as the ultra lightweight bodies (.020) of the old days, which is what most people think of when they talk about lightweight. It does remove a little bit of the weight up high, but it's probably not something to concern yourself with too much unless you're at a pretty high level. If you're curious, just buy one and spray bomb it white, they're only $20. Compare it with the .030 and see if you can tell a difference.
LW bodies aren't so much about overall weight, but rather weight at the very highest part of the car, which can definitely be a factor when considering CoG. Whether or not it matters to you on the track is something you'll have to decide. There are drawbacks, they're not as durable and more prone to tucking during a race.