The Skinny
406 is the only officially monitored civilian frequency nowadays, though some EPIRB's in non-revenue GA aircraft in the US still have only 121.5 units, as do some private coastal watercraft. A/C worldwide are still required to monitor 121.5 MHz 'guard', VHF equipped commercial watercraft are still required to monitor 'guard' on channel 16. Any other monitoring is voluntary and local only.
All of the new 406 MHz units must be registered with the appropriate authotrities as all TX a serial number to identify each unit.
Some of the confusion relates to the different types of EPIRBS being offered now. Basic ones TX only a coded signal on 406 at low power. Burst transmission is used and may cease after 48 hours due to battery life expiring. Better ones have higher power but this also requires better batteries, better than that has built-in GPS, and the best for AC have all that plus a low power 121.5 TX for old-fashioned close-in DF work.
With new basic units it may take as much as 2 hours to get a position fix good enough to initiate more than a general response- I don't think those are approved for any types of AC. High-powered ones will be more readily captured by multiple satellite receivers , thus rarely have more than a few minutes before a 'position fix' is worked out. GPS-equipped units have a response time measured in seconds. The 'local' 121.5 beacon may be useful for pinpoint SAR locating in mountainous or forested terrain but is all low-power so as to not interfere with 'guard' monitoring.
The new 406 units are less likely to be unintentionally activated by a hard landing but always check before leaving the AC when you aren't sure. In some countries you will get the bill for the SAR efforts if there was no emergency! If it were me, I'd go with a GPS equipped unit for most areas, and if I was often over mountainous or forested areas I'd opt for the 121.5 option too (which is supposed to be limited to aircraft EPIRBs only). Wikipedia has good info on this which I have verified in the US through the FCC website.
The old HF 'guard' of 2183 kHz is no longer supported and the 121.5 MHz VHF 'guard' is only monitored by A/C. Everything except Military is now on 406 MHz only so that's what you need and indeed is required by law in many nations for some or all A/C and for all Maratime use beyond coastal waters. Something is better than nothing if you've legally got a choice, but I tend to value my hide highly and would choose accordingly!