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.
Not true. Look at the various Skype clients, e.g. for Linux. Memory usage ballooned after they wrote their Electron based client. It uses 4 to 5 times the amount of memory of the old Qt based client and idle CPU usage is through the roof, too. And it can't even handle long conversations smoothly.
Absolutely equal no, or else it would be the same app. But that's contrived.
My point is that given two apps that both have the features a user considers "must have", few, if any, would pick the Electron one over the native, precisely because of the extra burden in cpu/memory/speed of Electron.