Talon and Zed

when one is banned or someone picked the other

I guess it's up to personal preference. Talon generally performs better than Zed, but Zed has more flashy plays.

If your team already has quite some stealth, you'd probably better pick Zed. (But since stealth is usually in assassin kits, it would probably be better to take a non-assassin as having 2 assassins in one team significally lowers your winning chance).

Note that being good with one doesn't necessarily mean you're good with the other as their kits are different and thus require a different approach and both require deep understanding in what you can and cannot do. Even though they basically do the same thing, they're played in a different way in terms of micro management.