I have pretty much the same view as Pinkymingeo. I wanted to include some powder checking system to my LNL in addition to eyeballing it, and the powder cop struck me as simply pretty much the same thing as eyeballing it. And I would have to make sure I look at it every time. That makes it much more vulnerable to operator error, to distractions. A few months back I had a couple of squibs, which simply tells me that the old mark-I eyeball, by itself, isn't foolproof.
I ended up buying the RCBS Lockout die. Very impressed by it. Like Pinky, I'll probably get multiple ones eventually, but so far I've used it with both .45 and 9mm, and to great effect. I'm still eyeballing as I insert the bullet, but having the extra advantage of the lockout die appeals to me very much.
To me, there's no contest. Lockout die wins.
BTW: If you get the lockout die, you might try something like this: I took a couple of small corks and cut them to specific lengths and stuck them into a couple of cartridge cases (no primers). One was "correct," i.e., i defined it as the proper powder load. The other was too long, and thus emulated too much powder in the case. And, of course, I had one w/ no cork, representing the squib load.
I used them to set up and learn how to use the lockout die. Works like a charm, and no muss, no fuss.