I paid a visit to my local outfitter and was provided a comparison table (assumed to have been prepared by ACR) that was very helpful. Apparently some ACR rep left it there so take it with a grain of salt:
cost of ownership over 5 yrs
ACR: $499-699
Spot: $644-1517
purchase price
ACR: $499-699
Spot: $149-169
Base fee per year/5years
ACR: na/na
Spot: $99/$495
Fee for realtime tracking
ACR: n/a
Spot: $49
Fee for $100k coverage/yr
ACR: n/a
Spot: $8
note: in the US, a fee is not normally charged to be rescued. 406 PLB owners may purchase it on open market.
Fee for policy for 5 years
ACR: n/a
spot: $608
Satellite system
ACR: COSPAS-SARSAT
Spot: Globalstar
Satellite type
ACR: GEOSAR, LEOSAR
spot: globalstar low orbiting
Antenna
ACR: 1 for GPS, 1 for distress message transm.
spot: 1 for gps reception and distress
Transmission Power
ACR: 5W
spot: 400mW
Note: ACR PLB has a 12.5X more powerful transmitter
Transmission type
ACR: GPS (lat/lon), 406MHz (Doppler), 121.5MHz (homing)
spot: GPS, 911 Alert
Battery Chemistry/life/warranty
ACR: Li+/5 yr off/5 yr
spot: AA 1.5 Li+, or Alkaline/1yr 0n/1 yr
GPS Failur backup
ACR: Doppler
spot: none
Homing freq
ACR: 121.5
spot: none
Emergency routing
ACR: Air Force Rescue Coord Ctr to local SAR
spot: SPOT GEOS call center, then local SAR.if no gps, only emergency contacts will be notified.
Sources: SPOT Website, SPOT data sheet, ACR website, ACR data sheet.