It's a good question, but I think it's like a "Ford vs. Chevy" debate. Let me explain...
Some people will use a cow call, because it is quieter (in case of elk nearby) and is non-threatening. They will start with 2 or 3 cow calls, wait a couple minutes, and the send a locate bugle. If no response, they move on.
In the early season, I don't think most bulls care about cows. Most of them could care less and they know there aren't any cows in estrus. So a cow call is not an "action". The bull, even if nearby, just thinks, 'oh... there's a cow. Who cares!" And the continue to lay in their bed.
But bulls, even in the early season, want to know "who" all the other bulls are. Most of them know each other. So when you "bugle" into an area, and they haven't heard you before, they will come investigate. It's not a call of "action", but you will still get their attention and curiosity and they will likely come looking out of curiosity. To size you up since you're the new kid on the block.
When hunting really thick areas, bulls are only likely to hear the highest note of your bugle. It has to be LOUD. The low part doesn't really matter in my experience. To some, it might even sound like an aggressive bugle - a level 7 out of 10, or even a lip bawl or sorts. But these callers are just trying to make the highest note to illicit a response.
Some guys will let out a short bugle or bark, prior to their bugle, to get their initial attention. Think about a bull feeding, or walking and 'whishing' through the brush, or dozing of in his bed. They may hear your bugle, but it catches them off guard because they weren't in an ideal position to REALLY hear. So a quick bark or something leading up to the bugle, followed by a pause, so they can turn their head, raise their head, quit walking (think.. they are echo locating, too) may allow them to hear your intended bugle which is the 2nd sound you make.
In thick country, I am amazed at how close one can be to the elk (or the caller), but because of terrain features (ridge, creek, thick timber, etc.) is is completely washed out in a very short distance. This is why its beneficial to bugle from ridges (you can bugle from each side) and to bugle on different parts of the mountain. I have bugled in spots where I thought the elk could hear me, but have to go all the way around and then I get a response. I don't think they were being lazy on my first bugle; they just didn't hear me until the terrain features were right.
I think the area and setup is also crucial as to how you "bugle". If you're in a known bedding area, and you're in the heat of the rut, maybe you can sneak into the peripheral of the bedding area and instead of locate, you use a bull calling cows bugle or something different. In short, the level of noise you try to create should be balanced by the situation.
I don't think you can really do anything wrong. You should develop a setup, cadence, and strategy that works for you. That will build your confidence. And confidence kills elk.