sirius3,
3-CMC is definitely more speedy and gives a strong rush after you snort it. It's accompanied by a moderate euphoria initially that slowly fades away as you keep redosing. The main downside for me personally is a pretty uncomfortable comedown and residual stimulation that makes it hard to fall asleep afterwards. Obviously you may combat that with benzos, if you're into them.
3-MMC on the other hand is way more euphoric and subtle in terms of the stimulation. It comes up and down gradually, so the rush is limited but so is the comedown. The peak can feel somewhat similar to MDMA, especially when taken orally. The dopamine is also there, making you energetic at the same time. Sleeping is relatively easy and you don't have to wait for it long after the last dose wears off.
Note that I might be biased towards 3-MMC as I simply like it more. Nevertheless I encourage you to try both.