After writing this message, I realize this thread is about a USB based distribution.
Yet I'm posting about a CD based solution in case it serves the ones who are still searching and deciding...
Here's how I did my setup to be able to exchange bitcoin and litecoin.
I don't want to use two computers for separate offline-and-watchOnly clients.
After searching and reading, I decided to use:
Electrum and Electrum-LTC from a CD-booted operative system,
rigorously offline (I unplug the ethernet cable from the PC so I
still have wifi for my phone), with a portable wallet on a pendrive
and a watch-only Electrum client in Windows (this can be
fully portable so I plug the pendrive just when I need to use the client
and nothing strictly related stays permanently on the PC which is often online)
I grabbed the latest Electrum (1.9.8) and Electrum-LTC (18.104.22.168) clients from electrum.org (the Linux versions come with executable binaries, too) and I added them to the current slax distribution (with the necessary modules but not the electrum one, and the startup command launches from inside the electrum folder) and made a bootable CD of it (although it works offline, it's good to know that it cannot be modified like a pendrive-resident environment).
IS THAT SAFE?
As long as the offline part stays offline and as long as slax and Electrum do not contain malicious software which might store my seed phrase somewhere in my hardware, that's pretty safe... apart of course the Electrum servers considerations etc. etc. etc..
If I come to get very rich (but I doubt LOL), I'll start using the Tor network (I already have the connection setup in Windows). (*****)
I saw that another slax based solution was out, I had tried BTCVault, but
the Electrum client is a little old
because of that, I didn't spend more than a couple of hours on it but
I couldn't have Electrum create a new wallet on the pendrive (and in
volatile storage either) through the GUI and the right-mouse-button
contextual menu item "create new wallet here", sure I'm missing
something; I could have it create a new Multibit wallet but that's
not my choice for now.
I have tried two more live CDs, Tails 1.2.2 and Ubuntu 14.10, but the boot time is much greater than with slax, and anyway I need the online watch-only part to run on Windows, and in the offline part of the process I wouldn't use the extra features that Tails and Ubuntu might (or not) offer over slax.
(*****) The only advantage of Tails which is evident to me even for offline use: Tails wipes the memory before shutting down... I don't know how much time I should wait after shutting down slax, before switching on the PC again, no idea.
I hope this helps some other crypto currencies newbies!
With the first "crypto-bucks" I will hopefully earn (I'm offering online lessons, trumpet, rhythm, musical theory :D ), I think I shall have to donate to the Electrum and slax developers, and I promise I'll try to avoid dust transactions and will not be stingy with tx fees.