I personally use the gatling and volley gun combo as you are an apc you are ment to be carrying troops and while we lack long range fire support the missiles, battle cannon and auto cannons are okay choices. Your mileage will vary with the second of the two options and I normally use the first of the two and if I am setting up for deep strike then I will also slap an augur relay on there for good measure.
However my standard loadout is always the volley and gatling combo as most people play marines along with camo netting for more survivability, which has saved me or many an occasion.
Going back to the long range fire support option you could argue that the long range options are good when paired with camo netting well that is not so true as even if you have that in standard cover well that is all fine and dandy until you look at the AV on the thing. So even with that cover save you will be making a lot of them because of the low AV which means you were dead to begin with thus its a waste of points that could be used else where. You need to be moving to deny you opponent from hitting the taurox in its weak points as a long range firing platform this is kind of hard to do.
Plus even without all of that you are not using it for its main purpose transportation, granted this does fit the fire platform well to but it is a mobile up close one to me not a long range artillery cannon. Once this drops its troops you have two options, one harass the enemy with the taurox or two stay with the troops and support them by being a fire magnet/mobile cover.
To me the key to getting the most out of the thing is my principle for everything I use no matter the army. What makes the unit you are trying to use that unit? For the taurox armor or lacking so stay mobile, give it some camo netting, keep it in terrain, kill the enemy before it can kill you, not hard to do with the gatling volley gun combo either as you are already dropping troops close to the action and so you are close enough to pump out so many shots from the guns on it any marine player will weep tears of blood, gets worse when you have the 4 in the ground assault formation as now you troops are twinlinked to without the need of orders, meaning its gets even worse with a command squad nearby. So what else do the taurox have well its a fast vehicle which helps its mobility tremendously, it is a very mobile firing platform and can carry 10 guys inside with 2 fire points on each side! (can someone say flamer).
This to me is one of the most versatile vehicles in the game due to the above mentioned, again my standard loadout is the gatling and volley gun combo, but that is not to say the other options on it are useless they just for me at least vary to much on there effectiveness.
Hope this helps.