I need to get the transmission ordered for my new build, so this question has been on my mind too. This’ll be my first geared build for ages …
This is based on a notional 29″ wheel size, so ’tis not terribly scientific, but (and, always assuming the maths is actually right 🙄 ) in terms of gear inches you’ll get:
Option 1: 21 – 100
Option 2: 19 – 111
… So not much to be gained at the bottom end, but a bit more at the top with the triple. That’s not the whole story of course – there’ll be more overlap with the triple, but that might [also] save you hunting around for quite the right gear too.
Depending on the uppy-downiness of the ride, you might find that running the double involves more shifts on the front, rather than just leaving it in the middle for more of the ride.
I’m almost certainly going for the 3 x 10 option, as I do ride on road stretches between the back-country lanes (so the big ring’ll be useful), and I don’t ever ride trail centres (in fact I’m not even really sure what one is). South Devon, so lots (and lots) of short, steep hills / ridges almost everywhere.