I've considered myself both a Nidalee 'main' and a Riven 'main' at different points. Currently I feel I'm the latter.
From my experience with both champions this is my 2 cents:
Before nerfs, AD Nidalee was very easy to play. However, if you fell behind you were useless (much like Riven)
Riven on the other hand is very difficult to play, and if you fall behind you are useless.
Now concerning AP Nidalee I would say she is very difficult to lane with. Possibly moreso than Riven. As Riven the onus is on you to make plays and snowball, but you have enough tools to do that with consistency if you practice. As AP Nidalee you are essentially relegated to a spearbot unless you can get an early kill, at which point you can do enough damage to be an assassin in the mid game. The nice thing about Nidalee is that you always have that 'spearbot' fall back, but I would say her laning and early game is more difficult as a payoff for that.
Overall I think Riven is more difficult. Her teamfights take a lot of knowledge and mechanical skill, and her laning phase is pretty tough due to low sustain and the prevalence of bully champions in top lane. Nidalee has a tougher lane as AP, but win or lose lane she will be useful unlike Riven, and her teamfight is significantly easier to execute.