An useful, much needed post for so many newbies. Can i ad how i see all this, and how i use them.
IMHO the key factor to decide which one to use is "how much funds it will keep, and for how long". I use four of the named five.
MOBILE: for small amounts 0.001-0.5 BTC, to be possibly used in the next 6 month
DESKTOP: almost as above, up to 1BTC, for 1-12 month storage. Using it less and less, tend to be replaced by ever improving mobile wallets.
HARDWARE: for 0.5 -5 BTC storage, and or for a longer term, 1-3 years
ONLINE: NOT use at all. NO intention to use in the future. NO exeptions.
PAPER: for funds larger then 5BTC, and for storage beyond 3 years. The only one I recomend for anything larger then 10 BTC , or longer then 5 years storage.
MULTI SIG type much recomended if you go significantly higher/longer.
I think this concept of diferentiation is rather closse to optimum.
At least to me, and to my current understanding of all these security issues.