The article on testing tandems gives the nod to 700c's for speed and stability. Interesting. The theory is that a longer "contact patch" is better than a shorter and wider one.

Practically speaking, road bikes are usually 700c and MTBs are usually 26" but there are sub groups of each that promote alternative wheel/tire sizes. I used to commute year round on my MTB and would switch tires for winter. This past summer I agreed to do a triathlon and started to commute on my road bike as part of my training.

At first it felt weird and uncomfortable in traffic. It's an old 80's bike and downtube shifters aren't exactly ideal for commuting. However I quickly got used to it and now prefer my road bike for commuting over an MTB. I like the fact that my center of gravity is lower. I carry my stuff in a messenger bag and my back would sometimes get sore on the MTB. It's less of a problem on the road bike because the forward riding position spreads more of the weight to my arms and puts less on my spine.

Plus I go faster. Way faster.

Now it's winter and I'm back to the MTB. It definitely has more of a "plush" ride but I need to get rack on it and offload some of the weight I carry because the plush ride doesn't compensate for the sore back. I also feel less stable because I'm up higher, although I'm sure that's more feeling than reality. Riding higher also makes me a bigger target for those nasty North winds we can get.

To me an ideal year round commuter would be a cyclocross bike. They have the riding position I like, are relatively light, yet they're a bit tougher and can accommodate wider tires than a standard road bike.

I don't see a pure cyclocross bike in my near future but I can see selling my MTB in the spring and picking up a used hybrid with 700c wheels in the fall and someday putting drop bars on it.

Anyway, as far as wheel sizes go, everything else being equal, I think the nod goes 26" if you're dealing with uneven/rough terrain and 700c otherwise. I'm sure the 29er and 650b camps disagree with me but I believe 29ers are popular with larger/stronger riders, which I am not. In any case, the tires, gearing and riding position probably make a lot more difference in comfort and performance than the wheel diameter does.