The HX series is a Silver efficiency PSU, they've been around since 2009 but with refreshes to some of the models.
The AX is a Gold efficiency PSU, introduced mid-2011 I believe.
The TX850M is a Bronze efficiency partially modular PSU. They are recent model. Don't confuse them with the TX (no M) series, which is a non-modular version that's been around for years. You'll see TX V2 models now.
I don't know about Corsair's RMA policy on modular PSU's, but when I RMA'd a fully modular Silverstone Strider last year, they only wanted the base unit back and not the cables.
Which would I pick personally? The AX850 if it's within 20 dollars of the HX850. Or as suggested check out Seasonic modular models.