I'm nothing close to a PLB expert, but ...

First, you have to understand that the clear view of the sky is so that the PLB can get a good view of satellites - so that you can get a good accurate GPS location, BUT the newer generation of GPS chipsets do an AMAZING job of 'hearing' satellites even with quite a bit of vegetation overhead. I don't think you'll have a problem with that.

Second, modern 406 MHz PLBs actually have TWO methods of determining location. The most accurate is via the broadcast of the GPS coordinates - that can get to within 300ish feet of your location. The second method uses a Doppler effect with satellites to determine your location, which gets within 1-3 miles, and you have to wait for a satellite to swing around to your location, which could take up to one hour (probably less).

Third, that 1-3 miles might sound like a lot, but 406 MHz PLBs also have a 121.5 MHz homing beacon, which will help properly equipped & trained rescue crews narrow in on your location even without that exact GPS coordinate.

To my knowledge the SPOT only broadcasts the GPS coordinates.

Here is a GREAT description of how PLBs work on Equipped.org:

http://www.equipped.org/406_beacon_test_background.htm