thanks Tedster, good to be here, have been reading threads on this forum for years, thought I'd join in all the fun!
Rockyou makes another good point about Google Instant, that probably accounts for at least 10% of the "success rate" differential.
All I have to do is compare my Analytics traffic/revenue for numerous keywords that have the same position in Google and Bing serps to know that it's not worth my time monitoring my Bing serp positions - the payoff just isn't there.
If anything, Google is taking market share. My Analytics certainly attests to this reality, Google's share of my organic traffic has gained about 10% points over the past year.