I think that the cornering thing greatly depends on the geometry of the 29er. My FS 26er corners better than my Salsa 29er, but my Salsa isn't designed to be a corner carver. It's an all-day kind of frame, and I knew that when I bought it. Wider bars and a shorter stem help with its cornering. OTOH, the Niner Jet RDO that I demo'ed cornered like it was its job. That thing flew through turns. Both are good bikes, but they have very different geometries.... (On a semi-related note, I'll probably end up selling the FS 26er to help finance and make space for a FS 29er.)

Ride lots of bikes. IMO, it's the only way to start forming preferences. Test rides at shops, borrowing friends bikes, anything....