Powder, says my sister the former clothing designer and rep - is better for your clothes. Don’t know why, but she won’t use liquid.
I use about half the recommended liquid and an eighth cup of Borax myself. I use liquid because the brands I use come in liquid form and don’t come (or aren’t carried by the stores I shop in) in a powdered form.
A friend of mine told me that you should always use powdered detergent when washing fleece - something about the composition of liquid detergents causes the fabric to become harder and less pliable (and therefore lose some of its insulating qualities). After experimenting with both types of detergents, I think she’s right.
When I lived in a big buildng with the laundromat in the basement it was a lot easier to carry a bottle of liquid down rather than a box of powder without spilling. So liquid is better for that situation
Even now with my own washer in my house I always manage to spill some powder on teh floor. So liquid is still better.
How differently do they clean clothes? Beat heck out of me. They both do a fine job using 1/2the quantity the package recommends. So I buy liquid strictly for the handling convenience.
Powder has the potential to be a better cleaner because they can include ingredients that are mutually antagonistic. Such ingredients require water in which to attack one another, and so in a liquid detergent would only last hours or days, which wouldn’t be acceptable. So, to make a liquid detergent, they have to use alternatives, which are always a more difficult compromise. I worked with a lady whose previous job had involved researching these alternatives (for liquid dishwashing detergent) and trying to get them to work almost as well.
I always thought it was just marketing, and didn’t know that people had problems getting the powder to dissolve.