I already mentioned much of this on another thread recently...
In spirit, I think I'm a powder detergent user. It was good enough for my mother's Kenmore, it's good enough for my piece of junk BOL WP DD. Plus I like the long shelf life (handy if one has a bunch of different detergents). I also like the fact if/when scented, the powder scents tend to be more restrained.
But, unfortunately, I've been using liquids most of the time since at least early summer 2015. I have not bought powder detergent since that summer, and have used existing supplies only occasionally for quite some time. Liquid detergents have one huge advantage: they are so common. They are most of what gets carried (and probably some stores ALL that's carried). Thus the best deals/sales tend to be on liquids. I might or might not do better per load cost with powder, but with liquid I can be more assured of having my monthly budget balance properly.
The only routinely cheap powder detergent I see is All, which sometimes hits $3/box at my regular grocery store. But I have not been terribly impressed by it. I still have a small supply of my last box, which I use sometimes to help vary the detergents used, and I've noted the cleaning power does not seem to be as good as the Tide unscented (which hits as low as $5/bottle locally).