Well, if we're talking gaming companies, the actual games produced by said companies are a pretty important factor. Afterall, Sega and Nintendo weren't/aren't gaming juggernauts simply for the hardware they made.
In this area, Sony has Microsoft beat, not only in the number of games they publish, but also in the critical reception of the games they publish and the overall sales of these games (Sony has been on the top ten best selling publishers list consistently for years, often fluctuating between 2nd and 7th place, while MS only breaks the top ten in the 2-3 weeks surrounding the release of a new game).
Sony also sells more consoles than Microsoft and has more brand recognition than Microsoft due to the success of the ps1, ps2, and even the psp, which sits at 60 million units sold despite software sales being flat outside of Japan. Even SCE's "failure" outsells the 360 handily worldwide, coming within 6 million total sales of the 360 despite launching a year late at $599. They have managed to easily outsell MS in Japan and catch up to MS in Europe despite one of the worst launches in gaming history.
The only area in which Microsoft beats Sony as a gaming company is in its ability to court third party developers, and even that is becoming harder and harder to do as the ps3 takes more and more software sales away from the 360. And even then it's not so much Microsoft being better at it as it is Sony not bothering because their first party efforts are stronger than ever. They're becoming Nintendo-esque in this regard.
But ultimately, the prowess of a gaming company is showed off best by the games they make. Every successful video game company has had a decent stable of first party titles to thrive upon. Even the ps1, a console notable for its large selection of third party titles, was driven by first party efforts, with two of its three best selling franchises coming from Sony (Gran Turismo, Crash Bandicoot).