only you know the answer... what is more important to you?
killing hard enemies like bosses on islands more efficient in total? (take 50/50 prof / brut)
or
dealing maximum damage on shielded / healing enemies in PVP? (take 100% prof)
if i had no free respec and had to choose one for a long time (or forever) personally i would go 100% prof, since it matters less to do a little bit less dmg in pve while in pvp its much more critical
(also in pve at the beginning of the battle the 100%-prof-player is superior in dmg to the 50/50 brutality-player, however this player gets superior as soon as the enemy's HP are blow 50%)
or, you decide to avoid targts which are shielded / healing in pvp and just go 50/50 (but thats a questionable decision since almost every class has shields / heals etc and in pvp there are always healers in BGs)
i know its a hard decision