What is better more cores and less frequency, or more frequency and fewer cores?

Question

Server equipment

At the moment I have 12 cores at 2.4 GHz
. At the peak, when there are a total of 500 people on all server sites, the processor shows 70-80% load, the RAM is busy as usual 5 / 48 GB (as well as at the moment when there are 0 people on the sites, that is it is not important here)
But soon we will buy powerful advertising, and probably the number of visitors at the peak will be much higher, so, is there any point in purchasing another dedicated server with an 8-core processor at 3.4-5 GHz? or still choose with the same frequency of 2.4 GHz but 16 cores?
The sites themselves are like single simple games, written in pure php5.6, with very narrow functionality, that is, there are no difficult tasks