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on the value you want to transfer. This has been discussed many many times, here in the forum and reddit, and Trace Mayer in his bitcoin podcasts repeats it every time - search for it
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Generally:
for a couple of Euros/Pounds/Dollars (up to 100 maybe), it is ok to use a normal machine and wallet (or iPhone and Android). Similiar to your normal wallet for FIAT money. To then copy via USB, use a machine, that has been verified for malware.
When you have values in the 1000 Euros/Pounds/Dollars range, you pay more attention, and copy this stuff from a secure, unixoide (xBSD or Linux LIVE CD) air gapped machine.
At this point in time a cold storage wallet like Trezor or similiar has it's value.
When you have more than 10.000, then you need to spend some time into re-thinking your store of value. Trezor is not secure enough, cause we cannot see, what is "below" the device. Here you would need to have your cold storage setup along with multisig. Do it yourself, make it trusted, and reliably tested. There is no final solution.
Summary:
low value, little effort in securing your keys, high values, much effort securing your keys.
spend two dollars in security efforts for low bitcoin values, spend 1000 dollars for security when bitcoin values are above 10.000 Dollars, and yes, spend much more, when it's a million value. You are your own bank!