I've ridden both Flixbus and ICE extensively, had good and bad experiences with both and would say that they are very comparable in most aspects, except travel time. If I would find myself in a position with less money again, I would switch back to Flixbus (though the gap when you have a Bahncard 50 isn't even that big on popular routes).
I wouldn't be surprised if they would lose significant business in Germany when the new Munich-Berlin ICE track is opened at the end of the year. 6hrs (ICE) vs. 8hrs (bus) will suddenly become 4hrs vs. 8hrs.
Comparing both pricing models isn't really that straightforward, which is why I left it out of my initial comment.
Also, in a train you usually won't end up with damaged legs or other flight related issues and you don't need photo ID or be treated like you're a terrorist (patdowns/searching, in addition that your PIR records WILL end up at NSA HQ, no matter what), which is the reason why I do not fly.
Also, as a Bahncard holder, you get limited-time offers all the time. Just this weekend, I made a 500 km trip with InterCity trains that only cost me 25€ in first class (down from 160€) because of a voucher DB sent me via snail-mail. Once every few months, I will similarly receive vouchers like "10€ off a ticket purchase within the next 6 weeks" or "2 people ride, 1 pays for your next ticket purchase".