They basically mean the same thing, though you're not one to settle for general answers. I would favor the use of "true" over "the truth": true <=> false, the truth <=> a falsehood, a big fat LIE

The last two both mean the same ~ at least that what I deduct from it. But the first one have a slight difference. 'What you're about to see is the truth.' suggest that they watched something before and what they saw before was not the truth and that this is the truth Where as 'What you're about to see is true' implies that they haven't watched it and you are just telling them that what they're about to watch is true. Hope this helps you :)