I test drove both of them when I was looking for a EV.
Ioniq is unbeatable interns of value for money, comfortable and large enough boot. The best thing to remove the terrible “foot parking brake” and have an electronic one instead. It was quite even at motorway speed, not a lot of wind noise.
However, the interior is typical Korean , but boring and it is a car take you from A to B comfortable not excitingly
i3 on the other hand if you have little use of a 5th seat, it is the most “driver” EV I have even test, (never test a Tesla or Porsch, or I-pace). The interior look very roomy and the design feel futuristic. However, do not expect top spec 3 series premium feeling and despite everything is optional, i3 is still “lack of options” to customise unlike those 3er or even 5er. Wind nose is more noticeable than ioniq. Yet, if you want a sporty and gimmick EV with relatively lower cost, i3 is the only choice in the used market so far