First it depends on the supplier. Second make sure any henna you get is called body art quality henna powder (from whichever country is fine such as morocco, yemen, pakistan or india).

The sift is going to be a main difference usually. Body art quality henna used just for body art and can be used on hair is at least 3-5 times sifted and is very fine.

Henna for hair is usually not as sifted but can be up to 3 times sifted. It depends on your supplier.

Another difference is freshness. Henna for hair is not always as fresh. It might be last year's crop and perhaps sometimes it is just as fresh.
You just have to make sure you ask your supplier these important questions to help you make the right decision in your purchase. Hope this helps!