Mono = guaranteed ability
Half = more choices.
The answer? I've played both and I still don't know!
I think split also screws with your opponent's head a bit. They have to consider which bonus will be thrown at them going first in a round. Like will a flora stop kinichaw's ability and make it pointless to use her? Or will they stop kinichaw's bonus with raeth and win the round so they don't have to deal with +19 attack with their roots half?
And split is also used when one clan bonus complements another clan bonus, like jungo/huracan or pussycats/freaks (Or pussycats/frozn).
I guess it depends on which clan and the cards from that clan you're using. Some would be better off in mono like isatis, pilzken and raven (obviously right?), but if you were using just cards like brianna, victoria, etc, you might try half.
I like split, for that mind game factor, but the problem is those 3-1 splits