All my string orders to Rotosound begin with an apology for the time since the last one. I have put in orders, in the past, because I feel guilty I haven't bought any for so long!
To my ears, flats give (what I can only describe as) a bigger sound. I don't go for ear splitting treble. They are tougher on the fingers (finger in contact with more string area, I guess) but they are my sound. I do have a couple of basses strung with rounds - the two 'Stingray' type basses. But all my Fender type basses are strung with flats.