FYI, beyond being "inkless", both of these are marketed as "forever pens" or "everlasting pens" (and to me, that's the interesting aspect of these) — but the $6 one isn't actually a real "forever pen"; it's just a (silver-core) pencil.

From the $6 pen's Amazon description: "Working principle: the nib rubs on the paper to decompose the alloy metal molecules and leave them on the paper surface". In other words: this is a pencil. It will wear down over time, and need new nibs. (You could also tell by the fact that they sell replacement nibs for this "pen." A "forever pen" shouldn't need replacement nibs!)

IMHO the only reason someone would be interested in the $200 pen is the "forever pen" aspect. (As, if not for that, if you want an "inkless pen", why not just buy a regular, cheap pencil?) The $200 pen does achieve the "forever pen" claim, insofar as it chemically reacts to paper without the tip wearing away. In contrast, the $6 "forever pen" isn't actually a "forever pen" (or "everlasting pencil") as it claims to be. It's an impostor product, a GameStation 360 kind of thing. So it's somewhat of an invalid comparison to review these as two products in the same category.

Also, re: erasability, think about it this way: if the markings can be erased, then those markings are not caused by a chemical reaction with the paper, but rather as a deposit on top of the paper. A real "forever pen" is necessarily a non-erasable pen, because it's necessarily a pen that's doing something chemically to the paper, rather than leaving part of itself on top of the paper. (Which is also why they call it a "pen" rather than a "pencil" — pens are writing instruments that do something permanent to the paper, while pencils leave something on top of the paper that can be rubbed away.)