It's really a style thing at 1500 points and 2000 points. Personally, I tend to rely on my infantry more than armour. I'll take dreadnoughts and the occasional predator in a 2000 point army, and generally that means that I've got a good 60+ Space Marines already and the tank is just sort of filling a role gap that I haven't optimized a squad for (and generally, it's anti-armor duty.) Some people prefer to put much more weight on tanks, which is fine-- but Space Marine squads are able to muster comparable anti-infantry firepower (and in most cases, superior anti-infantry firepower) so the need for Predator Destructors and Vindicators are really rather limited. Whirlwinds have a place due to their indirect fire capability as well as the option to load them with Castellan missiles, which, in certain situations, are quite useful if you're of a more tactical mindset and less of a "chop 'em into bloody goo" mindset.
I can help you with the fluff aspect of it, though. If you're looking at a Battle Company, then consider the following:
You have 60 Tactical Space Marines, which always fight as Tactical Squads. You've got 20 Assault Marines, who can either fight in Assault Squads, bike squads, or provide crews for Land Speeders; and you have 20 Devastators, who fight either in Devastator Squads or provide crew for Predators, Land Raiders, Vindicators, Whirlwinds... basically, your heavy armour. Dreadnoughts are permanent armoury attachments to the Company, but in a chapter that has a fairly average number of dreadnoughts, there'll be at least one, and probably no more than three in a Company.
Companies do not "own" Predators, Vindicators, Whirlwinds, Land Speeders, Razorbacks, or most of the Chapter's Land Raiders. Most Companies have a single Land Raider at their disposal for the Company Command, as well as ten Rhinos and twenty bikes. Other, heavier armour has to be requisitioned from the armoury. While Razorbacks are transports in game terms, Company squads are ten-strong and don't fit inside them-- they're issued from the armoury for depeleted squads and for specialized missions. The Land Raiders that are not attached to a Battle Company are typically reserved for the sole use of the 1st Company.
Onto crew. Land Raiders require a crew of three Space Marines, one of whom is generally a techmarine (so you're really only down two Devastators if you're worried about keeping to 100 Space Marines in the Battle Company plus command.) Rhino chassis tanks generally require a crew of two Space Marines-- a driver and a gunner-- but Razorbacks and Rhinos are operated by the squad they are transporting, and have no set crew at all. Whirlwinds and Vindicators may, depending on the Chapter, be equipped with servitor loaders to speed up firing rates. Obviously, Land Speeders have a crew of two, so the most Land Speeders a Battle Company could put in the air would be ten. Some Chapters have limited numbers of Land Speeder Tempests, which are single-man gunships-- but most Chapters only have a handful of them and won't readily be fielded unless absolutely necessary, as they're rather hard to maintain.
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