Once the computer is booted a Persistent install is as fast as a Full install.
With a 4GB pendrive you will end up with about 3GB of persistence space.
A persistent install takes longer to boot, is not as secure and shouldn't be updated.
The Try/Install screen can be removed by overwriting syslinux.cfg with:
say Booting an Ubuntu persistent session...
append file=/cdrom/preseed/ubuntu.seed boot=casper persistent initrd=/casper/initrd.lz quiet splash noprompt --