Naturally, a Pokemon that wants to use Choice Band has to have a lot of Attack.
For example, Dragonite is a good Choice Band user because it has great Attack. But also, Dragonite's speed isn't the best, so a Choice Scarf won't be fixing much.
Naturally, a Pokemon that wants to use Choice Specs has to have a lot of Special Attack.
So for example, Chandelure is a good user of Choice Specs because of its great Special Attack. However, a Choice Scarf may not fix much because of unimpressive speed.
A Choice Scarf user could be any sweeper than needs a tad more speed to shine.
Keldeo, for example, has very high speed, but enjoys a Scarf to beat scarved Pokemon that are slower like Garchomp. Kyurem-Black is also a good choice Scarf user because its below-average (by a very little bit, mind you) can be fixed with it.