In theory a 29"/700c would be a bit better for commuting in general, in practice a 26" works just fine.

Here is my take in regards to winter,
29" wheelsets roll along smoother and have the longer contact patch, both help when you are moving and with float but, these wheels are at a disadvantage due to their ungainly diameter when you're moving slowly, starting and stopping. One area I can see a 29r being more comfortable is on rutted trails due to the lower angle of attack(don't hurt me gary fisher, i'm parroting what you told me to!) when rolling the small bumps from frozen footprints.

Depending on your 29r frame the bottom bracket may be high and the chainstays will for sure be longer than a 26r, a high bottom bracket is good for clearing obstacles on a trail but makes it harder to balance and corner. If the chainstays are longer on your 29r you will also have trouble putting power to the rear-wheel contact patch. Since you are positioned further away, this is good for 29r going uphill but is a negative asset on flat-ground which is loose, like snow on a road. The 29r will also have a longer wheel-base, this should help to make it more stable at speed, but will make it tippier than a 26r at low-speed. Also your 29r low-gearing/gear-inches will not be as low as a 26r.

Anyways, those were just some thoughts I had deciding if it is worth upgrading, my answer was no. I still ride a 26r in the winter and it's fine. If i was just getting into things now, i would go 29r though... esp since Nokian now makes 29r extreme tires.