science/engineering/rockets VASIMR engines and propellant

Normally, I consider myself the physics guy, but I'm coming up short on my research. I've seen various types of gasses suggested for fuel to ionize and make these engines work. H and He because they are so abundant, and Ar or Kr are the current ones being utilized in testing. Is it generally going to be better or more efficient to use a heavier gas?

My thoughts on this, is that you can store a heavier amount of Ar, for a fixed size of tank, than you can H or He, therefore your fuel weight is higher for the same sized tank and your Delta-V numbers go up. But it takes more energy to get the heavier atoms moving as well. Would an argument against liquid helium or compressed hydrogen mainly revolve around the tank size required to store enough of it for something like interplanetary travel? Or is there something here that I'm missing entirely? Since H and He are so light, will they produce higher exhaust velocities for the same amount of electrical energy put in to the system? Thanks guys.

Thanks King Neptune. Yea, that made sense in my head, but I wasn't sure if maybe those gasses would present problems of their own. Nitrogen is another that comes to mind, as it can be stored in liquid form, but I think Ar and Kr would still allow you to store more mass easily. Nitrogen also being fairly abundant in the solar system gives it so potential advantages too.

Muppster, I was trying to search the AD homepage earlier and didn't pull that page while I was fishing around. Thank you for the link. But that further compounds the question. What advantage would a lighter gas have on Isp and Delta-V I wonder.