Maybe this post will be the shorter ever of this blog, but I have to write something about it. Microsoft offer two version of their Debugging Tools: a x86 version, and a x64 version. Which one do you have to use?
On 32-bit OS, the answer is really simple: you don’t have the choice but to use the x86 version, as the OS will not be able to start a 64-bit executable. End of the decision tree :)
On 64-bits OS, it depends on what you want to debug. Unless you need to dig deep inside the Wow64 layers (and I never had to do that personally, but speaking about it makes me curious about what I can found), just use the x86 version to debug a 32-bits executable, and the x64 version to debug a 64-bits executable. If you mess up with this simple rule, you may encounter serious difficulties to get CLR information through SOS (well, it’s basically not possible as far as I know), have wrong data about common memory structure that Windbg can display, and maybe be unable to load your favorite extension.