the mkii works fine... the difference is the usb port, and a couple of extra buttons...

neither of which will be much use with the ld400... i'd go for the mkii anyway unless you get a great deal on an mk1

the pedal is useful for alot of different line6 gear and can be used as a midi controller on your pc with the usb.

no demos of the sounds that i know of... some are good, and some are just showing off.. meaning not really useful.

i tend to use the express with mine rather than the shortboard, because i can get by fine with less than 4 tones for the bass.