I've always used premix, pro's often use dry mix if they have a mix machine as it's cheaper for them. Not sure why more for you to buy. I presume because 50 pounds of dry mix makes a lot more mud than the 3 1/2 gallons that comes in a box. Not sure what sizes you're comparing though.
As someone else said, practice on a sheet of cardboard or preferably scrap drywall first to get texture like you want it. It will pattern differently on wallboard than on cardboard. Make sure you have plenty of mud so you don't have to stop and get more. Having big enough containers to make enough mix to do your whole job makes for the most uniform job. If you're using setting type powdered mix, you don't have that luxury unless you can apply it really quick.
A technique I use is break up what I'm doing into areas that are all contiguous and intervisible. Make sure you cover all that sub-area with one mixture. If it's too big for the amount of mix you have, do a partial layer over the whole room, then mix more and do the same again until you're done. That gives you the most uniformity. Especially critical is to have good pressure regulation. I have a welding gas type regulator I use that keeps the pressure within one psi or less. Varying pressure makes for varying texture.
To get the most uniform texture over the space, I usually do a horizontal pattern of about 1/3 of the density, followed by a vertical pattern, and then a circular or random pattern, trying to see and cover skip areas.
I don't know what texture you're trying to do, or match. If I have a choice, I do a 2 or 3 step pattern, with a large blob generating mix first, very sparse on the wall, followed by a normal 1/4" or so orange peel applied relatively thinly, and then a high pressure, thin mix that makes a very fine splatter. Those textures align with the spray pattern I outlined above. The final fine splatter hides the taping seam difference from clean paper field much better in the final paint job.
I have found this to give the most uniform and easy to paint pattern, but it takes more time than a single heavy orange peel, which you normally see commercial crews doing. It's hard not to get some different surface tension in the orange peel at joints, so they are harder to cover with paint. I also sometimes use random latex paints to cut the mud with instead of water. Great use of leftover or mis-mixed paints. This gives better adhesion to the mud, and keeps roller stripping to a minimum while painting. Then again, I backroll all my paint instead of just spraying, to get better adhesion. I put it on with airless, and then roll it. My preference is to get a better job, not an easier one. For the easiest job, just do a fairly thin mix, put on heavy orange peel, and then spray over it with eggshell or satin. Fast, easy, not as durable, and hard to clean. I prefer to get a better texture on first, and then prime and paint with semi-gloss. Just a different approach.
