Steven makes some good points, but there is more to know:
The ACR actually uses the government-run NOAA weather (and probably military) satellites, the SAR system piggybacks onto that backbone. Includes a cooperative network of government satellites around the world. SPOT uses a private, commercial satellite system that some will say means by its very nature is less reliable.
Yes the ACR has a transmitter power of 5 watts while SPOT only a fraction of a watt. But signals at the same power travel differently at different frequencies, plus the height of each system's satellites' orbits is different -- so the ACR has to send its signal farther. I don't know for sure how these differences net out, but I do believe that the net effect is the ACR signal is more likely to reach its satellite.
The ACR does have a beacon signal, but that is a "local" signal -- the idea is that the satellite signal will be able to get SAR within a few hundred yards of you, then once in that range they can pick up the beacon to get within 10 meters or so of you. But don't expect SAR to pick up the beacon unless they were first alerted by the satellite and responded in the area near you.
The ACR satellites are positioned such that anywhere in the continental USA and virtually all of the civilized world there is a satellite in the sky within line-of-sight (assuming not blocked by dense forest, steep canyon walls, etc). The SPOT satellites have spread orbits that move the satellites across the sky, and sometimes there might not be a satellite in range.
If you hit the SOS button, that signal needs to go up to the satellite and then be relayed back down to a response center. If the satellite that the ACR uses cannot relay your SOS then it "holds" it until it is in range of a response center and re-transmits, but my understanding is that the SPOT's satellites do not hold the signal, so it will only work if both you and a response center are in range of the same satellite for an immediate transmission.
FYI I'm an ACR owner. It is a serious tool optimized for getting out an SOS and having SAR response, a true PLB -- SPOT is not a true PLB but rather a communication device that can also send an SOS.