My experience with GPU Tweak is that it's very unstable, it caused me to to get grey screens and blue screens all over the place, this was about 10 weeks ago with the latest version. I have never used Trixx but people don't seem to have any problems with it.
If you want to do heavy overclocking you want the Vapor-X version, it has the best power delivery and cooling.
The 3GB vs 6GB video memory thing is a non-issue, you don't need the 6GB version anyway.
Have you had a look at the MSI R7970 Lightning?
I use Trixx currently with my 6870 and it's great. No problems like that at all, for sure.
I was really considering the Matrix because of the Digi+ VRM and 20 phase power design... the Vapor X is better than that?
3gb vs 6gb doesn't make a difference except making the card $200 more expensive. Noted
No, I would never even consider MSI for anything, see this
. Just based on others experiences I've read about with their AM3 boards catching on fire, and all that. I don't trust MSI. I don't know how their GPUs are though so I suppose I'll have to research it.