The thing MS fans seem to not want to realize is that the 360 doing so well was largely due to Sony's mistakes with the PS3. It was $200 more expensive at launch compared to the 360, a price advantage MS made sure to keep in place for the first few years of the PS3's life. Sony probably could have done a lot more to help 3rd party developers come to terms with the new architecture, something that lead to ~2 years of poor ports. MS has to be given props for making great decisions this gen to keep it in front of the PS3 (pricing, 1st party and 3rd party suport, and great marketing). However, even with Sony's mistakes and MS's good decisions, the PS3 is on course to overtake the 360 within 1-1 1/2 years. This speaks volumes about the strength of the PS brand.
Given the above, imagine how next gen will turn out given that Sony and MS will probably be launching within 6 months of each other (not the 1-1 1/2 years advantage MS had last gen) and with a launch price within ~$50-$100 of each other. This scenario is looking very good for the PS4. In terms of regions, I think Sony will definitely take EU by a good margin (partly thanks to GT), Japan will be probably be a tight race between Sony and Nintendo (Wii U may slightly edge it out), and Americas will be a tight race between MS and Sony (NeXbox may slightly edge it out). As for final marketshare, I see it playing out like this:
PS4: ~40%
NeXbox: ~32%
Wii U: ~28%
