The difference is one that is often debated as to how significant it is:
Reptivite has "pre-formed" vitamin A definitely usable, but possible to overdose the chams with
Herptivite has beta-carotene, which may or may not (that's the debated part)
be converted into vitamin A by chameleons
Vitamin A is essential to chameleons' health.
Vitamin A oversupplementation is bad and Vitamin A deficiency is bad

Here's a link if you want to see just how much debate takes place on this:
https://www.chameleonforums.com/vitamin-scares-me-12395/
This is
one of the reasons that you should gut-load your feeder insects. It is known that crickets do convert beta-carotene into the usable form of vitamin A.
In the wild chameleons are believed to eat approximately 50 different types of insects--which obviously don't all eat the same things.
That's why chams don't become vitamin or mineral deficient in nature---
variety of insects and
well-nourished insects---which is why it's best to feed a variety of insects.
Some are more nourishing to chams than others--some are too fatty to be used regularly.
While 50 different insects is beyond all but the most fanatically devoted cham owners, it's easy to provide at least 5 or 6 different types.
At the very least, since your cham is already showing health problems, it would be wisest to spend a few extra bucks now and get some calcium with D3 and a good multivitamin instead of waiting for the reptile show.
Chameleons hide their problems for as long as they are able--so they don't get eaten, in the wild.
When chams have a noticable problem it's time to act without hesitation.
I say this with compassion for your cham and for you, as it can be heartbreaking to lose a pet.
If you let us know what area you are in, someone may be able to recommend a competent reptile vet.
An exam by a good reptile vet is money well-spent and I would surmise that it could cost less than a "replacement" cham.
There will then be no need to speculate as to the exact cause of your cham's problem and, should we be right and the cause is found to be MBD, the vet can administer a calcium injection which will much more rapidly stop the progress of MBD and start the healing process.
Even if you don't want to take your cham to a vet, now (though you know that Kinyonga and I have already mentioned that it would be best), it's good to know of one.
So far, I haven't read anything negative about Reptaid and it sounds like a good product.
I haven't used it for my chams, yet--but I am a recent returnee to cham keeping, after a couple of decades away from them (though not from other reptiles).
Whatever things you decide to do, I hope that you will let me know how things turn out and I wish you the very best with your chams.