I think it depends, If memory serves; someone said that the initial strike may be just drop pods, but if they're there for a campaign, they are going to unload heavy armor.
don't forget, in the grand scheme of things, a single company/ war band cannot win a campaign in its own. However, vehicles perform much of the heavy lifting when marines are down on the ground. It's alright to launch them down, but getting them back to orbit to send them back down planet side is a waste of resources.
on the other hand, it is completely down to how the chapter/warband operate: iron warriors/iron hands would want to bring as much firepower while not sacrificing maneuverability and astral claws/raven guard would not wish to get bogged down with all the extra logistics and vulnerabilities that Astarte vehicles present.
in short, Depends on too many factors such as time, enemy composition, resources (fuel, munitions and such) available, and who is operating in the same theater. while epic does paint a good picture of how a chapter would operate with vehicles, they're most likely would not get into said engagement and, in this heretics humble opinion, would ere on the side of flexible, infantry heavy compositions. also great to note, air units would be a must take...and strategist cannot underestimate aerial superiority
as such, I have tried to enforce my 19th to only carry things that they can move without armor support: havoc without predators..
hope it helps.