What I mean is that front derailleurs were perfected decades ago and there has been virtually no significant progress since. Front derailleurs just work and there is no point in agonizing over which to use. I've seen ancient Campy Nuevo Records shift modern triple cranks on touring bikes without issue and MTB derailleurs on compact doubles. Remember that the only significant difference between a double and triple front derailleur is that the inner cage plate on the triple is deeper to better pick up the chain on the granny ring. In short, getting the ideal derailleur may improve shifting, but probably not by enough for most to notice.
So in choosing a front derailleur for a compact double or wide range double I'd consider the following, in order of importance:
- Make sure the clamp fits the frame.
- If you use index shifting be sure it's compatible
- A front derailleur designed for a compact double is nice because the curve of the plate better matches the outer chainring, but I can't feel much performance difference. If I already owned a regular double or triple front derailleur I'd just use it.
- On 50.4bcd cranks there can be an issue with derailleurs that have a "sculpted" outer cage, as in the photo on the right. These cages can brush the crank arm. It's safer to stick with derailleurs with flat outer plates.
- Modern derailleurs with flat outer cages are hard to find, though, so I use a Tiagra front derailleur with the Grand Cru 50.4bcd crank, but mount it with around 8.5mm of clearance between the teeth and the outer cage, instead of the usual recommended 2-3mm
- With this kind of setup, you need to adjust the limit screws just right, but if you're careful with the setup it shifts well and runs fine.