Well, b4 the price went up, it was better to swap for an alum shaft than to swap your u joints out, but for $250, it leaves more to consider.
if you can find a used one for $100, jump on it.
The MAIN reason for the alum shaft is to kill vibrations, and NOT for performance. Do NOT get one thinking it'll make you go quicker. Vibrations typically occur after a gear install, which would therefore warrant an aluminum shaft.
If you don't have any speed related vibration problems right now, I'd just swap out the ujoints and save the aluminum shaft for later when you actually need it. $250 can go to a lot more nowadays.