Hydrogen is not viable for CTB, it's extremely flammable.
Helium supply is dwindling since it cannot be manufactured, it's expensive.
The most common CTB gas nowadays is Nitrogen, which has infinite supply.
Helium is not yet very expensive. The problem is, that helium may not be pure. It could be mixed with oxygen, in order to prevent its use for suicide.
Hydrogen is flammable, but it won't combust spontaneously. One must just be careful not to use it in a small, unventilated room and keep fire away. The advantage is, that it can be made with a cheap, home-made hydrogen generator, and be directly used with an exit bag. (Search on this site for "hydrogen method").
Nitrogen and other gasses (like argon) usually are available only in high pressure cylinders, and you need special equipment for pressure reduction and flow control. If you have knowledge about how to use this equipment, then it may be the prefered gas.
So there are advantages and disadvantages for every method.
I recently read that helium is still the most used gas for suicide, but perhaps this changed.