In addition to ETTL, some notable features differentiating the Flex/Mini vs the Plus II are
A) hypersync and auto FP switchover - which is a big deal if you were shooting in daylight and wanted fill-flash AND subject isolation with low f/stops / or other cases where you'd want faster than sync shutter speeds (freezing certain types of action, overwhelming strong ambient light sources without the use of ND filters and stopping down beyond diffraction)
B) the ability to control studio flash output (if you were using certain Elinchrom and Alienbees models and purchased the corresponding PW ST4 receiver for the Elinchrom/ AC9adapter for the Alienbees)
C) a hotshoe and an attachment point for 1/4 tripod screws - which is useful since except for the 580EXII, I think none of the current Canon EX flashes have a PC-sync port, which is the only way to trigger flashes or strobes with the Plus II...of course you could save some money with getting a Plus II clone like the Phottix Atlas, which does have a hotshoe and a screw attachment point.
D) a USB port - so you can update firmware as and when it becomes available. Plus II doesn't have this but should work fine precisely because it isn't ETTL and ostensibly involves simpler one-way communication between the receiver and transmitter.