I commute to work on a road bike (6 miles) and last year I picked up a used MTB to ride the bike paths to work during the winter.

I met a man on trail riding a Surley Pugsley, you know the one with the monstrous tires. He raved about the large tires in the snow etc. I can't afford a pugsley but I was wondering if anyone has any experience riding a 29er in the snow. the tire has a larger diameter but will that necessarily help in the snow, especially hardpack trails? Is it worth the upgrade from a 26 to a 29er?

We get just as much snow out here in Traverse City/Kalkaska MI as you do in Minn. - or nearly as much (after a certain amount it doesn't matter). I ran 26 x 2.1 studded all last winter. I kept the tire pressure on the low side, and shifted down at or near the granny gears when plowing through the deep stuff. Spin like you're going up a hill. I never was fast, but I reliably made it wherever I was going. Wet snow above the pedals is neigh impassable anyway. Once it packs down, or if it's the light stuff you can do deeper. The Pugsley looks cool, but I can't imagine pulling all that drag around. My wallet would never take the strain anyway.

Last year in Minnesota The Bike Hub Coop had a Pugsley bike demo out at Lake Phalen around the time of the winter carnival. And I think they might still stock some - you could try it out yourself this winter.

I did go to it, and learned a couple things -
1. Comfy - Those big fat tires really do feel exactly the same as suspension
2. Really nice on show - The big fat tires really did seem really nicely on snow - to bad I didn't bring my mountain bike so I could have done a back-to-back comparison. :-)
3. Still no grip on ice - Those big fat tires still slip out from under you on sheer ice, perhaps a bit slower but just like any purely rubber tire will do.

What tires are you using right now? Because of #3, I don't recommend a Pugsley for doing anything in winter other than toying around - unless you want to make your own, nobody sells a studded version of the puglsey sized tire. :-(

P.S. I do rider a 29er in the snow (a 700c). I don't know that it's a huge difference from a 26" tire, though - both 26" and 29er tires come in the same widths. Theoretically the 29er is supposed to be faster somehow, but I doubt it's a very large difference. But both tires come in the same widths - the advantage of the Pugsly in snow is due to it's much wider tire, which you can't get aon a 29er anyways (as far as I know).

There is another buried thread about 26v29r/700c for winter riding.

Don't think you will gain much....

Tires: If you're fat, rock the skinnies. If you're skinny, rock the fats. Seriously, I got through the last two winters with 700x35 Kenda Kwick tires and never felt undergunned. Well, after the Christmas snowfall and the ensuing Fun Warts (©®™) I kind of found myself wishing for studded tires (and since have got a wheelset with them installed) but honestly, it's not that much an issue. A good set of semi-knobbies works really well; you won't be losing nearly so much traction as you would be in a car or motorcycle. You just don't have the horsepower to create the slip.

...

You also don't have that 3rd and 4th wheel to keep you upright when you slip, like you do in a car.

Always an option though!

Not so.

Last year in Saint Paul, just across the river from Minneapolis, I was spinning my way up a hill. It was kinda one of those "wow" moments. Spinning tire, rooster tail (small) of snow, from just pedaling in low. Made it up the hill without stopping though.

BTW, this was on my son's MTB. The John Deere with the road tires would have dumped me after the first round of snow that stayed down.

Well. Over in Shelbyville, you expect to see that sort of thing!

The 29'er has a slight advantage is some situations but is worse in others. In slippery snow where you might be accelerating a lot the 29'er is a disadvantage. For commuting would not make any significant difference. Put the widest ties you can get on the 26 incher. Get studded tires if you have a lot of ice to deal with. For hard packed snow and slush studs make little difference. Better big knobbies. For riding on snow and roads I like the widest small knobby tire that I can find. They roll a little better on the pavement and still work well on packed snow.

Skinny or wide, studded or not, they each have their advantages for winter riding depending on the conditions. Since on my commute there's always a pretty good chance at going over some ice I use studded tires. As far as width goes, I compromise by having a tires that are kind of in the middle, - a 40 mm.

I don't think a 29er has any particular advantage in the snow or on ice. For winter I use an early 29er from the mid 90s and I like it because I can share wheels and tires with my road bike. Anyway, I'd just find a good studded tire (carbide tipped) for your MTB.