What is the difference between "Om Namah Shivaya" and "Om Shivaya Namah"?

There's the only difference of position. Meanings of both are same : “Salutation to Shiva”.

Generally we use “Namah” after the God's name, if we want to say it directly.
Like : Om Ramay Namah, Om Krishnay Namah.

But ancient people used to think that if we want to impress the gods, instead of direct saying we should sing it. So they created poems and added musical Chhandas.
If you read Vedas, you can notice there's an unique Chhanda (Rhythm) for each mantra.

Sanskrit poems include Mantras, Shlokas and Suktis. Like any other poem, in a Sanskrit poem we can rearrange the words to set the rhyme.

So basically “Om Shivay Namh” is a dialogue or a direct saying.

And it's poetic version is “Om Namah Shivay” with rearranged words.
This poetic version is a song/mantra without “zero-change” in it's original meaning and it's more popular.

So you should use “Om Namah Shivay”.

However you can use “Om Shivay Namah” also because meaning is same and Shiva is a kind god, he won't mind.