I would say frag is more frustrating (and i'm surprise the most of you doesn't think so). I hate being killed without noticing how! It's worse than backstab cause it's more surprising (almost scary^^)!
What's more, when being rad, you know you'll die so you may either kill yourself before or kill your opponent. You're not totally dead yet and you can still do something, that's why I found it less brutal than frag.
And it's funny, when you got reverse damage, to imagine the frustration of the guy who didn't got his kill