Actually, QLSO silver has far less instruments than HQ Orchestral. The quality is higher, of course, but Silver doesn't even have a full string section sample . And it's not like one is bad and one isn't, they're both far above the quality of your average free soundfont.
Anyway, I've gotten satisfactory results out of both HQ Orchestral and QLSO Silver, however the sample quality in QLSO is much higher. I have not used GPO, though.
So Edirol gets points for having a boatload more instruments than QLSO Silver, but Silver beats that out with a very high quality. Keep that in mind when shopping around.