I know this is an old post, but here are my thoughts.
I started my collection with a 30 cal SpecWar 7.62.
My thought was that a 30 cal can would be more versatile with the different calibers I had.
The 22 can limits you to just that. If you are talking a 223/556 can you are still very limited.
So, my thoughts are 30 cal first then add as you need to.
The only problem is that it is a very addicting hobby.
I now have 12 cans in different calibers ranging from 22 rim fire, 556, 6.5, 30, 338, and 46.
Yeah, there is something wrong with me. LOL