Although they SAY they've fixed the "issues" with Onix, I'm still wary. EPDM tubing in radiant floors has a less-than stellar history. Although they say it's been "fixed" I've gotten private-backchannel field reports of strange black goo clogging the heat exchangers of boilers and even the tubing itself in fairly new installations. There may be as-yet-undiagnosed material compatibility issues at play. Installation is much less labor intensive than with PEX (yet the tubing itself is more expensive), but any savings implode and go quickly negative if for some reason you have to replace it.
I don't want to be bad-mouthing the product since I don't have any direct experience with it, only second & third hand stuff, but when faced with the same decision as you recently on a staple-up retrofit I went with PEX rather than worry about it. (And yes it IS a more expensive install, but nothing like a 2x multiplier or anything close.) A friend of mine with an ~9-10 year old system using Onix seems to be replacing it/unclogging it a section at a time, seemingly on an annual basis starting about 4-5 years in, whereas the first radiant zone I installed in my own home 15 years ago using PEX (Wirsbo, if it matters), has been completely trouble-free. Watts claims they've had 20 year reliablity out of even the older Entran versions of class-action lawsuit infamy. I dunno- maybe they have. YMMV
If it turns out in 20 years the noo-improoved Onix really works out (and I hope it does) well, then I spent too much. OTOH, what's 20 years of waiting & wondering worth? I'm fairly confident the PEX will still be hanging in there no matter what. It's your call.