Personally I don't mind which I use. It's all the same to me, I buy about 500 red potions tier 1 per trip, and fill up on HP/SP stones. If I run out of HP stones, I use the red potions, and vice-versa. So long as I can continue grinding in the fields, it makes no real difference. Money may be an issue, but finding those potions @ 15 copper isn't much of a task if you walk around Roumen long enough.

As for which you should use first, you hit the nail on the head in your opening post. It's more cost effective to use stones at the lower levels, and then this turns around as you level up. One should also take into account whether or not they can mass produce potions at a cheap price.

I just use whatever I have. o_O I'm so low of a level it really doesn't matter much for me, anyways. But, I'm sure once I get on up there (level-wise) stones will be the back up, and pots will be the main source. It just makes more sense.