What would be the differences between a persistent USB Live Session and a installed Ubuntu in a USB drive?
If I understand you, you want to know the differences between persistent USB and a full install USB.
Persistent Live USB: 4GB or more
First, Persistent Live USB takes less space.
One can create a non-persistent Live USB with a 2GB USB drive. To create a persistent Live USB you would need a slightly bigger one, say 4GB.
Second, Persistent Live USB can be used for installation.
A persistent Live USB is essentially a copy of the installation DVD. The files in the original ISO remains as is. Updates and future installations are saved in the space designated for keeping the changes. Say, you update Firefox. in a normal installation the old one is replaced. In persistent installation the old one stays as is, the newer version is located in the persistent virtual disk (within the USB) taking up extra space and sometimes creating problems, for example for kernel updates. Now if you install Ubuntu from this persistent copy into another hard drive the original version of Firefox in the DVD image will get installed.
Booting a persistent Live USB gives the students access to the "Install Ubuntu" icon on the desktop. Students can "accidentally" start the install process and delete the contents of the hard drive.
Third, Persistent Live USB is less secure.
There is no login process in the Persistent Live USB. The default user has admin rights and does not need a sudo password to exercise it. This means, a malicious studnet can boot a persistent USB of another student and easily access stored documents, uninstall or reconfigure applications etc.
Fully installed Ubuntu in an USB: 8GB or more
First, Installation takes more space than keeping the image of the installation DVD, about 5GB.
Once installed, the system can be updated, and customized. Uninstalling unneeded software will free up space.
Second, Hardware on which the full install is created is important.
The Live DVD image in the persistent USB is created with compatibility with most computers in mind. However, once installed on a specific hardware, the installation gets a bit customized for the specific components. This is important particularly if the computer has some parts that require proprietary drivers. Once these drivers are installed, the USB may not work on computers that do not have those specific hardware. Conversely, if the installation is done on a computer that does not need any proprietary drivers, that specific installation may not work on the machines that do need them.
Therefore, if the goal is to use the USB in different computers, a persistent Live USB may be a better choice than a full installation.
Third, Full installation is more secure.
Creation of an user ID and a password is required during installation. This password is needed for any administrative action. One can also set it up to require the password at login.
Also see Difference between LiveCD, LiveUSB, full-install, and persistence?
Hope this helps
Installing Kubuntu on a USB pendrive was a very attractive idea ... and I couldn't resist to do it. At the begining it worked OK, a bit slow but OK ( there is a performance bottleneck on the USB plug, so pretty much better USB 3.0 than USB 2.0 ).
After practicing a bit, upgrading the system, applications and so on ... I was excited ... I went to a tech shop and bought a hi-capacity pendrive (32 Gb) ... I was going to CLONE my Kubuntu main install ( root partition, home partition and swap partition ) on the new pendrive. It was a bit complicated but I managed to do it. I had to adjust Grub, fstab and not many more things ... and it worked, a bit slow ( my laptop hasn't got any USB 3.0 socket ) but worked.
Besides the slowness, I was very happy with my new pendrive ... it got my highly customized Firefox (more than 30 extensions) , my tuned LibreOffice , my custom KDE effects, Thunderbird (with several IMAP and POP accounts) , Dropbox ... everything ... it was pretty nice. In order to mitigate the slugglishness I fine tuned preload parameters (it was already installed in my main install) , I modified a bit fstab so that /temp , /var/lock , /var/log and /var/run went to tmpfs. I also made a Grub profiling and things got a bit better, not comparable with a hard disk install, but a bit snappier than before. By the way Nepomuk and Akonadi were de-activated on my main install, so after cloning they also were no operative on the pendrive.
The initial planned use for my pendrive was homemaking, tinkering and disaster recovery lifeboat.
I was able to upgrade the system, the kernel, applications ... everything went smooth.
But there was one factor I haven't previously thought about ... ... ... the limited amount of write cycles a nand-flash cell can survive before getting corrupt.
After some time LibreOffice stop booting with no apparent reason ... some days later other application followed the same path ... some weeks later the pendrive died. But the first time it happened I sincerely was not fully aware what the problem was ... so I 'decided' it was a low-quality pendrive ... and I bought another (different brand) ... and I repited everything ... and after some weeks the pendrive began suffering the same problems. Then I run badblocks utility and it became more clear ... the nand-flash cells were getting corrupt after some weeks of use ... what a pity !!!
I pretty much prefer running a fully ultra-customized pendrive than a generic persistent USB live session ... but this was a real showstopper for me. I guess a persistent USB live session will not suffer this kind of problem due to I believe there is less I/O activity in that configuration.
On the other hand USB flash installs sometimes has some little problems other USB installs (HDD or SSD) do not have ... regarding with Hibernation and Suspending to memory.
I would consider two options : internal drive (HDD or SSD) or USB external drive (HDD or SSD).
Regards.
Using a persistent USB "LiveCD" has some disadvantages in my experience:
On some systems you can get error messages while booting, making the boot time dramatically longer, or even preventing the computer from booting altogether (my computers all do this, unfortunately :p ). I have not found a solution for this (yet)
There will be a default user with sudo rights and without a password (it can be disabled, but it is not so easy)
Kernel updates can cause problems, since update-initramfs will trigger something with grub (or something :D), and that doesn't work because casper does not use grub (this should not break the system, but will leave the kernel unconfigured)
So if you are just having your class try Ubuntu out, this is a good way to taste the OS, but if you are planning on using Ubuntu full-time, you might consider using a full install instead (either on an internal or an external drive).