This is a great question and something I've been picky about for a while with EVs.
It's actually passenger space... cargo space, the Y wins no-contest... bigger rear cargo, massive under rear cargo and much bigger frunk (which is nice and sealed unlike the Ioniq where everything is open (they need to take a design cue from Tesla and Ford who make great frunks). The wheelbase of the Ioniq is longer than the Y so it helps with the passenger space.
Front:
Seats are similar but the Y is more of a laid back type seat (like a car) whereas the Ioniq is more upright seating. The Ioniq also does not have a center console that extends to the dashboard, it's open so people with purses (ahem... my wife) can put stuff on the floor between the seats that they can't do in the Y that would otherwise take up back seat space or their front seat foot space. The Mach-E is also a bit more open between the seats which makes it feel roomer.
Shoulder room and hip room feels wider and this may be due to that center console... additionally, the interior door panels (where the armrests are) are configured differently so that also makes it seem like shoulder space is better. Headroom is sufficient. and I'm 6ft tall and with the seats being more upright in the Ioniq and it's not an issue. Maybe the Limited with the panoroof has less?
Rear:
I "fit" more easily in the back row than in the Y (and that's with the front seat set for me). I noticed this when I would sit in the Ioniq at auto shows last year (because dealers rarely had models in stock to check out and I was not really serious about the Ioniq to deal with a salesperson). Due to the increased inventory... I went to a dealership last week, sat in both rows and on the passenger side and also test drove it (I had only sat in a Limited trim previously which has that sliding center console which makes more front space so I wanted to check out an SEL trim). My kids are also tall so I had them sit in the back too to test it out and they said it's similar to the Y... but the seats recline a little bit more (from my eyes it doesn't look like a bunch more but even reviews say there is quite a bit of recline but that might be in the Limited?). Rear seats also slide front and back which could be useful for people with young kids who want to move them closer.
I don't need more than 5 seats but I would like to have it just in case and just for overall space. I also want an R1S (tip: If you already have a reservation, you can get access to the R1 shop where you may be able to pick up an R1S sooner than 2 years... although still a lot of money).
But the build, fit and features of the Ioniq 5 make me hopeful that the EV9/Ioniq 7 are going to be great alternatives.
Ride:
I drove it for a while (dealer was an hour a way) so it was softer than the Y. I have some routes where the bumps were more noticeable so once I take those in the Ioniq I can report back with a better comparison.
Once I drain this down to 10%, I'll report back on what the actual DCFC charging time was to 80%.
One thing that was interesting is when I turn on the AC, it adjusts the miles remaining down to account for the AC usage (which happened to be a con because there is no dedicated button to turn off the AC... just touch buttons for every other function... also no dedicated buttons for heat seaters... so you have to go into the Climate page on the touchscreen to toggle AC and seat heaters (which drove me crazy in v11). Also, I haven't found a way to put shortcuts on the center screen but I can program one button on the steering wheel to a limited set of functions (but not AC
).
I only got it yesterday so still playing around with it.