I'm allowed access to Sentinels, Leman Russes, Chimeras, Predators, Rhinos and Trucks
Very cool list, the Cult, and outstanding vehicle options. But, each has their own uses, and although I'd probably shy away from some in my own armies, I do not run a Cult, and your needs may differ.
Sentinels -=- Ya know all of those gaunts that those Bug players love to field but not paint? How about those Str3 Space Elves (both white and dark meat)? Do you know how to piss off a squad of 50 Conscripts? If not, you have not read the first word in this paragraph. These units are very common to the game, and simply cannot harm a sentinel in close combat unless expensive upgrades are taken (and these rarely are). Sentinels average 50pts a pop and can carry a variety of useful anti-infantry/light tank weaponry. I'd not take Lascannons unless you plan on fielding many many Sentinels and are also upgrading them with HK missiles. Sentinels can also act like mobile terrain, being walkers, they can block LoS to more vulnerable units behind and also can block assault corridors, which may buy you more time to better defend your position or even stage an assault of your own.
Leman Russ MBT -=- Everyone knows that the Russ is KING, but not everyone uses them effectively. The Russ, in nearly all of its forms, is primarily an anti-infantry beast. If you keep this in mind, and only attempt to remove vehicles when better options fail to present themselves, then you'll command them just fine.
Chimera AFV -=- I didn't see 'Hellhound' in your list. Well, this will have to do. This little baby can max out at 27 shots (considering hull heavy bolter, turret multilaser, pintle heavy stubber, 6 lasguns, HK missile, and firing from the back firing point, a heavy bolter and a plasmagun), that is 27 shots that can decimate any infantry unit within 12 inches. Now there will rarely be a time when this opportunity will occur, and it is quite unlikely that your opponent would allow your chimera the chance. So, you don't give him the chance to silence one chimera. Make him try and silence 10! Backed up by a few Lascannons and such to help minimize the losses to your chimeras, and you have something to deal with, for sure!'
Predators -=- The 'true' tank fighter. Give this badboy a few Lascannons and position it in LoS of the enemy target, but in cover from the enemy infantry. This guy is pretty straight up, just watch your flanks, as infantry with meltaguns and such will most definitely make a bee-line for your rear hatch.
Rhino -=- Consider the cost, and the purpose, and you'll actually find that the Chimera and Truck can do the same, and be better at it. (this is, of course, assuming as well that the truck and ork truck are one and the same). This vehicle is OK in marine armies, but given the option, I'd leave them there.
Truck -=- Lets see, a vehicle that does not prevent assaults just because it moved 2" during the turn. BARGAIN. If you plan on fielding masses of mêlée crazed nutters, then get them some trucks so they can get stuck in and relieve those tensions! Just keep in mind that only fielding one or two will most likely see them twisted wrecks by the second turn.
In any case, the ubiquitous adage of "Take many, or take none at all." applies to vehicles as well. If you want your vehicles to play a dominant role in your army's battle doctrine, then you'll have to invest heavily in them, both time, money, and tactical knowledge. If you simply want to add another element of diversity into a predominantly infantry fighting force, then a few vehicles here and there can make for a very interesting presentation, and provide a few more tactical opportunities on the table top. Either way, I hope you enjoy your choices.
