In my opinion volibear is really strong, he can shred enemy adc/apc easy because his W deals so much damage! in other hand malphite can be a great in teamfights if they have some combo's like orianna, yasuo, it can be game changing!
Both are rly good tanks and have their roles in taem, if u need combo team then malph is better, but voli have more dmg also can toss back enemy carry and help ur team to kill it, Slight favoritism toward Voli