My advice would be to go with the ACR TerraFix 406 WITH THE ONBOARD GPS.
It is usually easy to tell which has the onboard GPS when ordering since most places have both the one without the onboard GPS and the one with the onboard GPS, and the later one is more expensive. The model without the onboard GPS is called GPS I - the I stands for "Interface". The model WITH the onboard GPS is called GPS I/O - the O stands for "Onboard" - it has both the GPS Interface and the Onboard GPS, thus the I/O.
Make sure you get the one with the onboard (internal) GPS receiver. If hell breaks loose, you're possibly severly injured or immoblized, and you're needing help, you simply do not want to be messing around with a seperate GPS and the associated pair of interconnecting cables (more on those later). Doug Ritter strongly recommends this too.
The MicoFix is a bit smaller and has the advantage of a one-button trigger, but the Aero/Aqua/Terra-Fix406 is still a GREAT unit and costs $100 less.
Put this $100 toward your Doug Ritter Mk3 "superknife" (I really like the Mk3).
Now as to the difference between the Aero/Aqua/Terra versions. The PLB unit itself is identical for each model - exactly the same. The AeroFix has a black holster case, the AeroFix's holster is blue, and the TerraFix's is green. Personally I wish the holster was a brighter color - maybe that neon green.
A more important differfence is that the AeroFix and AquaFix come with a GPS interface cable that has bare-wire ends, while the TerraFix comes with a GPS interface cable that has a 9-pin male serial connector that can connect to commonly available cables that connect to GPS units. The AeroFix and AquaFix also come with a small screw-in hook that allows you to hang it on a wall/bulkhead.
When made my own PLB purchase I didn't know about the interface cable difference. I bought the Aero version from Aeromedix.com - mostly to support a great company (Aeromedix has treated me real well). I was surprised and kind of disappointed to find that the AeroFix (and the AquaFix) only come with the bare wire hand-held GPS interface cable, so I had to order a separate 9-pin serial connector interface cable, which cost an additional $37 plus shipping (sigh). Yes, I bought the PLB with an integrated GPS, but I still wanted to be able to hook up my high sensivity GPSMAP 60CSx to it if necessary - if the PLB had a problem getting a GPS fix.
Had I read Doug's review more carefully ahead of time I'd have known that only the TerraFix comes with the 9-pin serial connector - he points that out very clearly at
http://www.equipped.org/terrafix_406gps_plb.htm . My mistake.
The TerraFix's interface cable runs from an optical connector at the PLB to a 9-pin male serial connector. If you want to use the interface cable you'll need to make sure you have a cable that goes from your GPS to a 9-pin female serial connector. My 60CSx came with a USB cable, but not a serial cable, so I had to buy one too.
If you're going to be using the PLB in field - as opposed to connected the PLB to a plane or ship system - and might bring a hand held GPS along with you, then do get the TerraFix instead of the AeroFix or AquaFix. Otherwise the three units are identical except for holster color, the style of GPS connector interface cable, and that little hook.
Ken K.