If you use a jigsaw make sure you get a metal cutting blade (the one with small teeth) The thinner or smaller the blade is, the more it will let you cut smaller circles or curves.
If you use a hole saw make sure you get the one with a regular drill bit in the center to keep it from walking all over the place while drilling.
When you use a hole saw on thin metal it can tear and bend the metal if the teeth catch when the saw is half way though. If you find the teeth are catching while drilling, just run the drill backwards at high speeds and push.
The reason I recommend the dremal is because I think it will give someone that is not used to using power tools to cut and drill metal the best results
I've drilled allot of stuff