Lightweight Spackle is probably Vinyl Spackle and yes it's like the Woodland scenics stuff but maybe more like flex paste. I think the light-weight plaster is based on the chemical formula that results when dry to be less dense and therefore lighter. Plaster can weigh a ton so lightweight is better.
Go to the store and get some small packages and try the stuff. Each has it's merits and you need to try it out for yourself. Don't depend too much on what we say. You might find yourself comfortable with something completely different. With hydrocal you can add pigment. With spackle you can trowel in details.
It's like working with any materials, try it out on a small diorama. I've done a lot of scenery in my time but when I get to that point with my latest project I'll try some new stuff on small bits. Don't spend a lot of money buying large quantities yet.