I want to say zinser because eveyrone else is. But the truth is, I would "grab" original kilz off the shelf. I'm used to the smell and the buzz. I think zinser is "stronger" and I try to keep in mind the fumes in a residential situation, how it effects the animals, furnace etc.
I'm sure there is a difference, but in my mind, not enough to argue about.
I think zinser is a product that leaves a nice "finish" to sand or paint, but I don't care. I like a thin coat. I think it costs more, doesn't make much difference in the top coat, and I can do the work. Don't need the primer to do any more than cover up what I'm priming. I'll do the rest, thank you.
It's like a drywall patch. One where you can still see the hole. You touch it but don't believe your own hands, it can't be filled. But if you just paint it, it comes out smooth. you can't believe your own eyes sometimes if you're a painter.