Ram Speed

Ram speed depends on speed and amount of channels

With one 3000 stick you're looking at single channel 3000Mbps * bus width so
One time 3000Mbps * 64 = 24GB/s theoretical max

With two 2400 sticks you're looking at
Two times 2400 * 64 = 19.2GB/s theoretical max

Therefore dual channel 2400 Vs single channel 3000 is a question of 2*19.2GB/s vs 24GB/s

Spoiler: dual channel is a lot faster xD

Now if the one stick runs out of space it has the put the data on your SSD instead, in case of a SATA SSD the max speed for that in theory is about 0.6GB/s which is a looot slower.