I have used both and liquid works better IMO. When I would mix pow. they would sometimes clump up and not desolve all of the way even when mixing over and over again.
I like the fact that I can have a small L container with a pour top and use a broth injector to messure the amount called for. Easy and clean.
I also would think that the liq being water sol mixes 100% with the water so you know that your plants will get the same amount through out. I don't know how acc this is.

The other reason is the desolve issue, I did a test awhile back and placed a bag of pow into 2l soda bottle, I would shake it up over and over a time period and the pow is still not fully desolved.