If there isn't much of a gap in raw power between the next xbox and PS4 I don't think the market will even notice. Sony have the talent and ability to squeeze a lot of juice out of their systems so a minor difference will probably be negligible.
If they release something similar in specs to the Wii U the two consoles will probably share a similar ecosystem of multiplatforms, and the Xbox could potentially be in a difficult position by being isolated (if they go ultra high specs). They'll also ship it at a much cheaper price. The downside being first party xbox games will look far, far better.
The issue of loyalty or the selling power of a previous console doesn't carry over as much as you'd think, I believe a new generation always has millions and millions of gamers on the fence waiting to see what's revealed. Some will buy into the franchises they prefer regardless of the horsepower behind them, others will want only the top end console, others won't be willing to spend too much etc.
I think releasing a moderately powerful console would be in Sony's best interests. Their strategy should be to continue the free online service while offering more incentives and value to PS+, continue building their first party portfolio and offering strong downloadable content and lastly to continue to improve and maintain strong relationships with third parties. If Sony can hit those points I don't think the strength of the hardware will be an issue.