I use both Sonar and Wavelab for Mastering, but mostly Wavelab if I need to export to various formats where the Metadata is important. Wavelab also has a very nice metering suite. With that said, I'll Master in Sonar sometimes because I have 3rd-party metering plug-ins that work very well, and it's easy to go back and re-tweak the mix if I need to. Global Freeze is worthwhile to save some CPU time. If I'm mixing the session as well, I will work in both realms (which is ostensibly blasphemy to most Mastering engineers).
Chuckebaby makes a great point. When mixing, don't even consider Mastering. Just get the best mix you can possibly get, and worry about Mastering later. So, I mix my sessions without Mastering on the fly. However, I may drop in some routine Mastering chain plug-ins into the Master buss where I can enable/disable them to see if there's anything in the mix that may have adverse effects on the future master. But I only use that chain typically to burn a fast demo for an artist, along with the regular mix. I always end up Mastering later with better results. So, this sets a good level of expectation.
As far as plugins in the Mastering chain that I use, they're mostly 3rd-party, but careful use of the built-in plug-ins can garner you a pretty darn good Master. The newest Multi-band EQ and Compressors offered by CW are great for these. The 4K compressor is wonderful on the Mastering buss. Careful use of that is phenomenal on drums.
I posted my approach
here a few weeks ago.