I've tended to use powders in laundromat front loaders, and liquids in top loaders generally. Am presently using a liquid (Seventh Generation, smells slightly of oranges) because I can measure it out accurately. (It seems to me that powders are subject to changes in volume relative to weight, as a function of ambient humidity and being "fluffed up" during measuring.)
Also using a measuring glass that looks like one of those little glasses that's used for drinking some kind of distilled spirits, but is calibrated in ounces and milliliters. This will be useful for comparing the quantities of various brands of liquids. Also allows me to pour the stuff into the wash water and then dunk the measuring glass a few times to get all the detergent out, before starting the agitator.