Which needle is next? Here's how I think of it. The stitches are all connected just as they would be on a straight needle, so the next needle is the one with the next stitch, you wouldn't skip ahead and start knitting on a stitch that isn't next in line.
I use magic loop a lot. I have used two circs but I have this problem of using the wrong needle and ending up with a mess. No matter how I determine to concentrate and pay attention, pretty soon I've used the wrong needle and have to go back and get things sorted out and in the right order again...that's especially true when I'm knitting 2 of something at a time. I prefer magic loop because there are fewer moving parts, others like 2 needles better, it's a matter of personal preference, or as in my case, reducing the options and opportunities for screwing up.
Are circs a substitute fore dpns? I guess they could be thought of like that but to me they are another way of doing the same thing. Anything that can be knit on dpns can be knit on circs and vice versa.
Disclaimer: I've not used dpns much because I had circs and they were easier to cope with. The very skinny dpns I used were dangerous to my health, safety, and well being. LOL However, I did just buy some sz 7 dpns thinking they'd be useful for i cords and holding sts....and they were on sale very cheap.