i don't know how is it with 2,25s or bigger but i've ridden 1,9s and 2,1 and i can surely tell that 2,1 is a lot more comfortable and has a lot better grip i didn't notice any difference in rolling resistance it's the same story as with roadie 20 and 23s but i think that a lot depends on tire's material, shape and manufacturer. The 2,1 also seems to be less prone to snakebites I've ridden michelin's wildgrippers 1,95 comp xs 1,95 and comp Ss 2,1 the last ones are great their grip islike nothing else and through whole racing season i didn't got a flat on any race (some marathons and xcs) and i only got three while training (two nails - no tire would survive this) and one faulty tube. But after 4000km of use they ar a bit worn.
I prefer 1.95's. Guess it all depends what you plan on using them for. I commute, hit fire roads and single track. If I planned on Hucking off bridges or downhill runs, then i'd want 2.4's. Either way I'd break my neck.
Let's see. I switch back and forth between Panaracer Fire XC Pro 1.8's and some old WTB Racing Raptor 1.9's on a narrow set of rims. I switch back and forth between Ritchey Excavader Front/Elevader Rear 2.1's and WTB Velociraptor 2.1's on a second set of wider rims. Once I make a swap, the tires usually stay on until a situation comes along that simply demands a change.
The narrow Panaracers rule on narrow but generally smooth singletrack. They are light, fast, and hold a line very well. I'm suprised how well the front tire hooks up in the corners. They do better than expected in sand and mud but are clearly out of their element. At the preferred 55psi the rougher roads really start to hurt my ****. I save them for when I really need the extra speed.
The Racing Raptor semi-slicks were closeouts bought on a whim for riding the Moab Slickrock Trail. I run them anywhere from 45psi on slickrock up to 60psi on the road. They are good on the road and stick to sandstone, but on a lot of the local trails they can be downright scary. The front wheel and sometimes even the rear will just let go. They are worthless in the sand and mud. Being closeouts, they started to split from dry rot after the third or fourth ride. They'll be replaced soon enough, but I usually have them set up for commuting.
The Ritchey's are my newest tires at two months and they seem custom designed for most of our local trails. They are a skinnier 2.1 and are comfortable enough on the rough dirt roads and rocky trails at just under 50psi. They have a perfect balance of traction and weight they never seem to wash out in the loose gravel. Being 2.1's, I would expect them to be better in the sand, but the lower knob height just doesn't get the job done. They tend to pack up with mud too, but I live in the desert so that's rarely much of a problem.
The Velociraptors were what came stock on my hardtail. They are bigger 2.1's with giant blocky knobs and are pretty hefty at 700g each. These are my mudders and sandbox tires. In spite of the big knobs, I find I have to keep the pressure under 45psi or the front tire will just lose it on sandy hardpack. This makes them painfully slow on faster trails ridden with faster friends. After four years, they were starting to show their age when I picked up the Ritchey's. They are now kept in reserve for the occasional sand traps and mud bogs.
Each tire seems to have a terrain it was made for and a pressure it runs best at. Narrow tires are usually faster but less comfortable and more prone to snakebike. Wider tires tend to be more comfortable and offer more traction at the expense of speed. Lighter tires of a given size often lack taller knobs, have thinner treads and casings, or are a soft compound that sticks better but wears out quickly.
I consider having a quality set of tires that matches your riding style and the local terrain one of the best upgrades you can make. Avoid buying closeouts, many of these tires didn't sell for a reason. Once on the clearance rack, they've often been on the shelf long enough that dry rot is a distinct possiblity. I think tires are one place where you should just bite the bullet and pay close to retail.
There is no such thing as the better tire size. You pick the most appropriate one for the right conditions. Wider tires will generally offer much better traction, cornering, flat protection, and bump absorbtion at the expense of weight and higher rolling resistance. They will accelerate slower and require more power to keep them rolling.
I run 29x2.1s (IRC Mythos (F&R)) on my Gary Fisher Mt. Tam_29. These roll and climb great for my riding purposes (purely recreational). I frequent hard pack trails and some sand in the spring/ summer/ fall, and snow in the winter (when it's not too deep).
The larger sizes are good for rough, rocky terrain, because you are less prone to pinch flatting and rim damage. We have lots of sharp, angular limestone in Utah.
I run my bike on 2.0 and find that i get great traction on the dry dirt but as it rains and the dirt turns into mud my rear wheel ends up spin and spaying mud up my back. I need to buy two new tires some time in the furture and i get some time different.
i ride 1.95 becuse that is all the bigger i can stuff inbetween my forks and stays with out worying about tire rub
and the wider tires will actualy have a lower roling resistance in the 'loose-loomy" condisions due to the fact that they coast across the top instead of sinking and they will have better traction almost all around
but i always ride cross country so i dont need anything to "fat"
When I was riding a lot in the 80's the style was a big 2.1 up front with a skinnier rear like 1.9. Now I think I have a 2.1 and 2.0 but like skinnier 1.9 or 1.95 and will probably go with them next time.