Both have advantages and disadvantages. For today's discussion, let's just focus on the problems kiln wash presents and how to overcome that.
Kilnwash (a fine powder that's mixed with water) firstly, is the cheapest way to lay down a separator. It's disadvantage is that it is usually applied with a brush making the surface smoothness dependant upon how the wash has been mixed, the quality of the brush (usually haik) and the skill of the 'painter'. Regardless, we usually end up with brush strokes which will then be embossed into the backside of your glass when taken to high slump/fusing temperatures.
A mouth atomizer at just a few dollars is cheaper than a brush, and is so easy to use. The finish is virtually flawless. Here's a short (and sorrty, a bit boring) video on how to use one.