From memory, Derby also won the toss and choose to bowl when we battered them in the QFs. They clearly haven't learnt their lesson.
I get the advantage of knowing what you need to score by batting second, but it takes away the opportunity for the opposition batters to face scoreboard pressure in a run chase. OK, 265 is a stupidly high score, but Somerset were always capable of getting something massive. So their only tiny, tiny chance was to bat first, hope to post something decent (eg 200) and then hope the pressure of the chase got to Somerset. By putting Somerset in, Derby just gave them license to cash into a shockingly poor bowling attack.