interesting question but since we have both a ex30 twin and ex40 twin (xc40 recharge) in our household, I see the following differences:
1) ex30 is way smaller than, hence lighter
2) body material is less sturdy, hence lighter
3) interior material is thinner and recycled, hence lighter
4) less buttons, speakers and padding, hence lighter
5) ex40 twin or extended range single motor have 82kwh, hence it will be heavier
Yes the ex30 is faster but I can't express in words how the ex40 is better at every day driving. the ex40 gives you much better control, especially in braking terms.
The ex30 is a next gen EV with no ice sibling, so the drag coefficient is way lower than the ex40 or ec40. As other said, the ex40 still has ICE remnants in it while the ex30 does not. It doesn't mean that the e30 is flawless (like practically non existent OPD, but that's another discussion altogether), the worst problem with it is the flat/skateboard battery structure which makes rear passenger arch their legs upwards.
Hope the above helps, I'd gladly answer any query the original poster might have