We are always going to be in need of, I think so at least, two different browsers... While working on windows, my primary choice is Internet Explorer of course; my secondary is Safari. While I am working or spending time on my home PC, which is a Mac, I use Safari as my primary and Opera as my secondary.
For Windows I use Safari for secondary because of I believe I must have same browsers on different platforms to accept something as, you know, baseline some-kind.
Ok, but why am I not using Opera as my second on the home PC so I still can have a baseline? Then again users who has and utilizes two browsers at the same time of course use their primaries more often. And if I use opera as baseline and the secondary at both environments, my usage will be small portioned (for Opera) and I will be missing the, you know, best of both worlds. Don't take me wrong, Opera is a great browser, but for mobiles you know.
Only if MS Internet Explorer was on Mac too that would change everything...