Your pricing is wrong. You can get an EPIRB
for @ $400.
GPIRBs start about $100 higher. You can also get 2 PLB's for about $400 (stick to the McMurdo's they are less expensive).
As for the choices, no one can answer that for you but you. I wouldn't head out east in Maine without some sort of beacon (EPIRB or PLB). Plenty of others have sailed the world without one. For my way of thinking, they are just too cheap today to forego.
I don't see AIS, tethers and beacons as interchangeable in any way.
The bottom line
though is Mum, right? Seems to me its a no-brainer.