The choice is actually more interesting. Although big companies such as Slack potentially have resources to support development for multiple platforms, even they can implement new features faster due to Electron. Think about all those small teams that just wouldn’t be able to port their apps anywhere without it.
So in some ways I’d probably prefer a good Electron app.
For this, a mail client, I doubt there'd actually be much saving going native - it's going to need browser level HTML either way. So here Electron may have negligible cost. Thunderbird easily goes over 300M when it's been running a while for instance.
For other things I'm far less keen as Electron is often a sign of a needless memory hog - 100M pomodoro timers and such like.
So in some ways I’d probably prefer a good Electron app.