I use both for different things. Opera's built in VPN is quite useful to me for bypassing reddit bans and I get the peace of mind of knowing for 100% certain that it's on and running each time I click a page. I've had situations where locally running a VPN has toggled off without noticing which is disastrous, having the VPN icon as a clear on screen indicator that it's running for the next click I make is really important to me to ban evade and not fuck up.

They both perform roughly the same. I personally find Firefox's URL autofill really fucking annoying though because it autofills at the bottom of the list so instead of pressing down once like in Chrome or Opera(chrome2) you have to move the mouse and click or press down many fucking times. It's bad UX.

Opera does something fucking weird and annoying with closing a new window that looks like a tab and it makes me mistakenly close all windows when I only wanna close the one tab I unintentionally opened. That's bad UX and fucking annoying too but less frequent than the Firefox URL thing. It has caused me to wipe entire comments I was writing once or twice though.

Firefox is probably better for privacy, I doubt the new Chrome stuff will be removed from Opera.