I would not say the lancers are better.
Hence, 'arguably better'. I'm not convinced that they're better
myself, but some of the testing on the current and previous generation of Lancer mags has been suggestive, and I've certainly seem the claim a few times. (E. g.: there was an article a year or three back in Shotgun News that got mentioned here. Drop testing was involved.) PMAGs have a longer history and a very good service record: they tend to remain at least somewhat functional, even after being damaged. OTOH, I haven't heard of anybody damaging a Lancer in a significant way, yet (I'm sure it's happened
, of course: I just haven't heard about it). I think it's at least debatable, and different people may reasonably draw different conclusions from what data's out there.
And, if you're looking for transparent AR mags, for whatever reason you may care to name, Lancers are more-or-less the only option available right now.
FWIW: I have a mix of different mags for my ARs and ACR. PMAGs (in various colors, revisions, and with and without windows), Lancer L5s and L5 AWMs, a Tango Down ARC, some USGI mags from a couple different manufacturers (all with Magpul followers), a Cammenga EasyMag. Of what I've got, I like the PMAGs best, with the Lancers a very close second and the GI mags right after that. YMMV.