Comparison between a Full install and a Persistent install to flash drive:
Advantages of a persistent install:
1) You can use the persistent pendrive to install Ubuntu to another computer.
2) A persistent install takes up less space on the pendrive.
3) You can reset the pendrive by overwriting the old casper-rw file with a new one.
4) The install to pendrive takes less time.
5) Slightly longer lifespan, a flash drive can take over 10000 writes, this can take years or decades.
Advantages of a Full install:
1) You can update and upgrade.
2) If you have problems or wish to modify, the solution is the same as with an internal install, (You can ask for help in these forums).
3) No ugly startup / install screen.
4) Better security, you can encrypt home folder.
5) You can use proprietary drivers.
6) Hibernation works.
7) A persistent install is limited to a 4GB casper-rw and a 4GB home-rw persistence file, to get more persistence requires persistence partitions, (possible with a mkusb persistent install)
8) Faster boot.
Note that once booted, both methods run at about the same speed.
I have used Full installs for many years but have been using a mkusb persistent install lately very fast using the run in RAM option.
Performance differences between Flashdrive install and HDD
A SSD would be fastest, then HDD, then USB3. If operating in RAM a flash drive should be fast enough.