In my opinion, Diana excels at basically wrecking the enemy adc's game. Fed Caitlyn..? No problem! Fed Diana..? Big problem..
Diana is a lot riskier to play, with no real escape mechanism. Fizz on the other hand does less burst damage, but has quite good utility.
Yes diana is quite viable even after her nerfs. She is no longer OP like she used to be, she is quite balanced in the current Meta.
If you are concerned about skill, Fizz as a character is quite easy to play, and requires only good positioning. His ult can at times be hard to pin to the right person, but it is almost always guaranteed to hit somebody. Diana on the other hand, is very very easy to play, but very hard to play well. You need to be good at judging exactly how much damage you can do to your target, and then you must pick the right time to deal it. A bad judgement will almost definitely lose your team that teamfight.. :3
In the end though, to me it really comes down to which i enjoy playing more, which is definitely Diana.
Just my thoughts on it