I am the proud owner of a 26" front suspended cromo Kona Hardtail Team Bike from the mid 90's as well as a modern 29" front suspended cromo Hardtail GT Peace (geared, not SS). On paper these two bikes have an awful lot in common. They are both multi-geared ridgid-butt steel mountain bikes with suspiciously comparable angles. This makes them perfect to compare 29 vs 26 in the rubber-meets-the-dirt world.
Let's start with bumpability. The GT just goes over stuff like a hijacked tank. Head on obstacles require less body english than the Kona, but we knew that already thanks to the magic of YouTube animation. The 29'er is also easier to keep at speed due to the increased wheel circumference. It likes to keep motoring along. The 26 just doesn't roll as fast or as easy, but while the increased wheel circumference likes to keep going it also tends to keep the GT going straight. The smaller Kona wheels with their smaller diameter steer quicker and feel more nimble.
In fact, the Kona feels more nimble all around because not only does the increased circumference of the 29" wheels slow handling, it also puts added weight in the worst possible place. 29" wheels have more rotational weight at the outermost edge of the rotation than a 26" wheel does. Added rotational weight means that every time you spool up or slow down those big wheels need to overcome added inertia. I am no weight-weenie, but don't poo-poo this one. When you accelerate or decelarate everything on the bike speeds or slows together, but wheels ALSO must spin. Saving weight at the wheels matters more than weight on the frame, and the Kona feels like it accelerates quicker.
Finally, the added wheel size of the GT requires more frame material and a longer wheelbase, putting on weight and adding to the slower feel of the GT. Slower handling isn't all bad, though. The advantage is that the GT feels more relaxed, less squirelly and generally more stable and sure-footed than the Kona.
There are two things that I didn't notice. First, 29" wheels require more energy to stop, but that didn't show up in the actual performance. Second, a 26" wheel is stronger than a 29" wheel but I have not had reliability issues with either so we'll chalk both braking and strength to non-issues for this test.
So here is the bottom line. For tarmac and hardpack roads, rails-to-trails, West Coast fire roads, jaunts to the coffee shop or an all-day tour it would have to be the stable and faster GT 29.
For technical mountain biking, East Coast switchies, competitive rides or a faster workout it would be the Kona 26.
So which is right for you? The one that gets you out riding.