What's different between the two? JetBrains lists some differences here, but I heard there might be some inaccuracies in the list.

My answer does not claim completeness but here is what stood out to me.

ReSharper C++

Pros:

• Has static code analysis. I especially liked that it marks potentially "superfluous includes", although this is not always correct.
• Gives the impression of being more feature-rich than Visual Assist.

Cons:

• Twice as expensive as Visual Assist and when your maintenance period of one year runs out, you only get to keep the version from the beginning of that year.
• It seemed to me that it did not work well in combination with CMake. I think it always did a complete rescan of the code after re-running CMake. During the rescan, not all the functionality is available for all files and the scan slowed down my system.
• At work, I develop on a virtual machine and and here it was unbearably slow. At home on a native machine it is better, but still slows down Visual Studio during the rescan.
• Did not work well in combination with the VSCommands plugin, so VSCommands must be disabled while using ReSharper C++.

Visual Assist X

Pros:

• IMO significantly better automatic code completion than ReSharper C++.
• I liked the refactoring functionality better. Can't remember why ...
• Half the price of ReSharper C++ and you get to keep the version from the end of your one-year maintenance period.
• IMO gets not as much in your way as ReSharper. But ReSharper has lots of settings so this problem can possibly be fine-tuned away.

To be fair, I should mention that I bought a license of Visual Assist and only used the one-month trial version of ReSharper so I am a little biased. The problem was that I knew the important shortcuts of Visual Assist already when I tried ReSharper so working with ReSharper felt slower and less effective. I seriously considered buying ReSharper but the slowness in combination with CMake made me decide against it. The only feature from ReSharper that I really miss is the "superfluous includes" thingy I mentioned above.

I tried ReSharper about half a year ago (end of 2015). Both programs can be used freely for one month as a trial version so you can just try what works better for you.

Edit:

• The information about the pricing may not be accurate any more.
• In the meantime, buying a Visual Assist key has become more complicated which is somewhat annoying. Now a human must confirm your purchase which may take to the next workday and you additionally have to register the software at another web-site which requires setting up an account.
• I think the trial phase for Visual Assist has been shortened to weeks.