So over the years I had gotten to ride few different setups over various terrain while bicycle touring and bikepacking in North America and some other places. While I am no expert, here is my take on how these bikes handle, feel and ride.
with 700c Opus - Legato: ~13 000km, 4 months (mostly paved)
with 26" Surly Troll: ~30 000km, 2 years (probably 40% on pavement)
with 29" Niner - Jet 9: ~6000km, 3 months (~10-15% on pavement)
with Fatbike: Fatback - Rhino FLT: ~2000km, 3 months (10% on pavement)
700c
This was my first touring bike, it handled quite well even under very heavy load. For the most part I rode pavement, aside from ~700km of dirt on The Dempster Highway, where it would definitely have been much better with bigger tires but even with 32mm it was manageable. I think if you are strictly going on pavement (with occasional rail-trail or well graded dirt road) 700c is the way to go.
Parts availability is good, especially in countries like USA/Canada, further down it may get tougher to find replacement tubes, etc. but all big cities would have at least 1 or 2 bike shops that would have 700c parts
26"
This was more of "thinking for the future" as I knew eventually I will need to install larger tires. I started off with 2" schwalbe marathon mondials and did a good chunk of dirt and pavement. I swapped to 2.35 nevegals for the Great Divide Mountain Bike Route which on a fully loaded touring bike is still doable. In winter. The only issue I had was when roads got really rough I kept breaking bolts on the rear rack. Not so much an issue of the bike/wheelsize but that is when I switched to rackless bikepacking setup, with front suspension.
Without bolts to worry about and a heavy load to lug around, things really changed. I was able to do a lot more roads and trails and it was definitely more fun.
Parts for 26" are the easiest to find and the cheapest, available nearly everywhere but like with other more uncommon bike types, if you need to replace an important part you will likely need to go to a bigger town (e.x. wheels, hubs, bottom bracket..)
29"
A stark contrast to riding the GDMBR in the dead of winter with a touring load, bikepacking on the full suspension Niner with an ultralight setup was amazing. The larger wheels roll over rough terrain much easier and with the full suspension you are invincible.
The bike performed amazing both on rough roads and on pavement. Parts are surprisingly easily available in Latin America with a good mark up. The biggest problem for me, from a touring perspective is the maintenance and parts. With constant cycling, the front & rear suspension need attention every few months and the XX1 drivetrain on it was a little too pricey.
FAT
In spring of 2016 I spoke with a friend in Calgary who was planing to build a fat bike and go ride it in iceland. Although iceland is out of reach, the idea of a fatbike stuck and after some time I was able to get a Rhino FLT frame and mix and match old and new parts for the rest of the bike.
With the kind of riding I tend to do, going fat seemed like a great option. The drawbacks are the following:
-harder to find parts and definitely a lot more expensive (eventhough capital cities in Latin America would have fatbike parts, it's almost easier to get those shipped from USA or elsewhere). With 60mm rims, I can also fit a regular MTB tire in a pinch.
-slow. If you end up riding on pavement a lot you will probably hate yourself. At this time I have only had short stretches which feel amazing after spending all other time on rough roads and trails.
Now, a fatbike just does a lot better on rough roads, really making the difference between type 2 fun and type 1 fun. Especially when things get ugly, you always have a plan B. It is not a "cheat" code and you will not float over everything but given the same road, you will suffer considerably less than if riding another bike.
forget the 29er or a fatbike, you need a mule..
Aluminium vs Steel
having spent 2 years on a heavy steel bike I recently switched to aluminum and absolutely love the weight savings, especially when you need to drag/carry the bike. No need to worry about rusting either but I'll report on how aluminum does in the long run. Now, especially since I run a light setup, I feel that Aluminium would be the best choice.
carbon is a little out of reach but it would work as well unless lugging a heavy load. Lots of people race the GDMBR on carbon bikes.
So, my 5cents on the bikes:
700c - if you are riding pavement with some rail trails and you know you won't end up doing anything more. This will shine with full 4-pannier load.
26" - most affordable and reliable choice. Works well bikepacking or touring, pavement or off.
29" - absolutely amazing riding, definitely better than 26" and can ride great on pavement if needed
FAT - I think on par with 29" but if you know you may end up doing gnarlier stuff, fatbike is better. It also forces you to always look for the harder roads, which always ends up being more interesting.
availability of parts: eventhough 26" is widely available, you still want to use a higher quality parts from a bigger town (or by mail). I think that makes the parts issue a little less important, especially if you do some planning in advance.
So my opinion is, unless riding purely on pavement - go with a 29er, it is the most flexible choice. If you plan on doing a lot of off-road and rough trails, snow, etc. a Fatbike is even better, especially if you are not in a rush.
but sometimes it doesn't matter what kind of bike you have because you will need to carry it anyway...
