Speed distribution of lighter vs heavier gases
To my understanding, the speed distribution is wider for lighter molecules. The Wikipedia page for the Maxwell Boltzmann distribution has two images at the top that convey this very well. 


In the first plot, $a=\sqrt{k_bT/M}$ where $M$ is the molar mass and $k_b$ Boltzmann's constant. For a Maxwell-Boltzmann distribution, the variance is given by $$\sigma^2= \frac{a^2\cdot(3\pi-8)}{\pi}=\frac{k_b\cdot T\cdot(3\pi-8)}{M\cdot\pi}$$ So, this shows that the speed distribution will be wider for higher temperatures and smaller molar mass.