I joined the Empire, because of the opportunity for fast advancement. With Lord Vader choking the top brass on a daily basis, the positions open up quickly.
Sure the potential for retirement is slim to none... but you gotta look at the big picture.
Imperial Army- Shack up with some hot babe on a Super Star Destroyer, quick advancement, power, glory, and no visions of dead dudes popping up all the time.
Republic Army- Bottom of the barrel Hoth outpost guard porta-john cleaner.. no advancement, no hot babes, all the power of a scrub brush, removing someone else's daily "glory"