Seems that both work on the MX cams, according to Panasonic.

Yes they are both OK, I have tested both, and I will say, "Buy the cheaper PL, the CIR-PL is not necessary". Again, at risk of being labeled "hypocritic", I am using CIR-PL. ;) Just because I bought it without test driving. These forums sure are useful!

That information took about 4 months to get from Pana, and after at least 10 e-mails.

I use circular ones, and they work fine. But it is good to know that circular ones aren't needed on these cams (and most cams, if not all).