I have used both and I have seen good results from each. Reptile relief sprayed on a rag then rub down the snake does a great job of getting mites that are moving around. It also works well for cleaning the cage of mites, and riding them from the enclosure. Though once the stuff dries it is useless. Those mites under the scales are a bit tricky so I like to soak the snakes.
Provent-A-Mite is great and simple to use, within a short while after the snake has been put back into it's enclosure I have seen dead mites on the paper towels(which I like to use to be able to see the mites).
I still use both, though provent-a-mite works the best of the two.
Reptile relief is a product that needs to be used more then once to treat the snake. Decent for the price.
Provent-A-Mite is a use once product. Spray the enclosure and it is good for a few weeks. I have not had to use P-A-M more then once. Very pricey, so I use a similar(exactly the same stuff with a different name and can) but cheaper product. REPEL Permanone, it is available in the sporting good section of WalMart for $5.