Actually most long distance riders ride ordinary road bikes, more likely endurance models. Carbon frames are very common, panniers very uncommon. Everyone pretty much tries to keep the weight down and frame compliance up. That said, there are still gearing differences in that one wants to be able to easily spin up most long grades on a particular brevet. Keeping leg effort down is a key objective.
You can figure what low gears would work for you personally by doing a little research. First thing, take a possible bike out and ride a long grade on it, riding at an effort that you could keep up for a couple hours. Note your speed. Then go to an online gearing calculator and see what gearing you'd need to be able to spin your most comfortable cadence at that speed and add one cog below that for bail-out. There you are.
There is no answer as to what's best, only for what works for you within your budget.
My personal preference is for 3 X 9 or 3 X 10, 26T small ring, paired with a cassette which gives me the gear-inches which I need at the bottom. That gives me the closest possible gears in the low end, which I've found to be helpful for me on long pass climbs, the key sections of a long distance ride. Anything works well on the flat. Other people have different priorities and the answer will also vary with gearing availability on a candidate bike.
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