What's the difference between double and triple fronts? Is it primarily the greater maximum capacity?

I'm planning on using a TA Cyclotouriste crankset and researched my way around to learn that the extremely narrow clearance between the arm and chainrings on these things means that they're unfriendly towards shaped cages. And I'd been hoping to pair it with Ultegra front and back (this is for a renovation of what I think is an old Jeunet frame… would like to keep it tasteful but not necessarily historical, starting from raw steel f & f).

The Ultegra double has a flat cage front and I've seen mentions of these working with Cyclotouristes run as doubles. The triple has a shaped front. So I was thinking I'd maybe just run it as a double. And then I started thinking that if I ran it as a 48-40-32 triple it'd be just at the max capacity for the double (16t) but it wouldn't have such a big drop and I'd get the nice mid-range. Or I could run it as a 48-32 double. Or I could get a different derailleur.

Any thoughts on which would work best?

You can use triple derailleur on double chain rings and vice versa. But what matters is the shifter, if that's STI type, you have to choose one that'll works with the number of rings you have.

You are correct that shaped outer cages don't work with the Cyclotouriste because of the tight clearance. Velo Orange sells an FSA with a flat outer cage that will work, but he doesn't say that it works with a triple. You could ask him.

https://www.velo-orange.com/fsagofrd.html

The FSA looks more or less exactly like the Ultegra double. I've got both spindles so if the one doesn't sort out I'll try the other.

I was wondering more generally about the purpose of the deeper back cages on the triples… is it just extra capacity or is there some other advantage?

Started poking around the V-O blog site and came across this little bit, where he says the deeper cages helps them upshift from small grannies.

I'm running 42/28 cyclotouriste rings on a Nervar crankset with a late 70's Campy Record double front derailleur. It shifts out of the granny great. I had to replace the original Schwinn approved front derailleur because it wouldn't shift out of the granny very well.

On another bike I'm running a Campy triple with an early 70's Campy Record double front derailleur. It works great as well. The teeth difference between the big cog & little cog is 16.

Based on this, I'd think the main difference is just the teeth capacity. My mountain bike has 42/32/22 triple on it. That is is 20 tooth jump which would be over the capacity of most doubles.

My experience with the doubles is in friction. YMMV with index.

I was hoping that was the case.
I was planning on bar-ends where (I think) the front is generally friction even when the back is indexed. Will probably try to stay full friction.

https://sheldonbrown.com/front-derailers.html

i think your 48-40-32 would work very well with a double.

you encounter the biggest problems when you try using a triple FD when your chainrings are close in size...like your current 8 teeth jumps. the bottom of the inside of the cage is going to rub on the 40 when you are in the 48. you can raise the FD so it doesn't touch, but of course you know the implications of doing that when shifting to the 48.

triple FD are great when you have huge jumps like on MTBs or a tourer.