Go for metal, either aluminum or steel. Carbon is sexy, but it’s not durable enough for a bike you’ll be riding all day for days on end, parking at gas stations and restaurants and campgrounds, and pounding over unpredictable terrain. Aluminum is lighter than steel but more expensive. Steel is easier to repair if you manage to break a weld.
You can change a bike’s performance quite a bit by changing its tires. To have the most flexibility, get a front fork with enough clearance for the biggest wheel and tire combo you might want. The wider a tire, the more stability you’ll have on rougher roads. But by increasing the width (and traction), you’re decreasing efficiency on pavement. If you want to primarily go faster on pavement, choose thinner tires (30mm to 50mm).
If you thought climbing hills on a bike was hard enough, try adding panniers and camping gear. More weight means you want lower, or easier, gearing. A single or double chainring should provide what you need while saving a little weight. Your lowest gear should be between 20 and 25 inches (that’s how cyclists express “easiness”).
You’re going to spend a lot of time on your handlebars. And while cyclists love to debate the merits of dropbars (like you see on most road bikes) versus flat bars (like mountain bikes have), only one thing matters: comfort. Dropped bars are more aerodynamic and offer more hand positions, while flat bars place your hands farther apart for better control on bumps, and they allow a more upright body position that many riders find more comfortable.
Accessory Attachment Points
You need plenty of mounts for attaching panniers and other bags. Look for small threaded mounts in the metal; the more the better, as they give you more flexibility in outfitting your bike for packing a balanced load.
Stretching out the distance between your pedals and rear axle distributes the weight of your panniers better and puts them in a place where they’re less likely to bang on your heels.
While you can turn anything into a touring bike, a full-suspension mountain bike is going to be heavy and inefficient for road touring. But consider a carbon fork—it will be better at absorbing shock than metal, transferring less of the bumps into your hands.
A Good Fit
Nothing else matters if the bike isn’t comfortable and efficient. Spend some time at a local bike shop testing out frames to decide exactly which size you need.