in previous Saphires event was farming champions to stay in top5% and it took quite a lot of time to do it while not buying ("saving") event cards for next month. So i will buy both 4k and 2k cards early to easy stay top 5% and dont spend so much time on those champions, because for sure people will be farming this event again very hard. i also advice people to think about buying those cards early if u want more rewards from event or to play event more relaxed way, i think its more usefull than 360 saved CP (3889 kobans) for next event
but ofcourse if u can stay in top 5% without extra points from cards there is no benefit in buying them early.