The plastic ones are just as "good" but the aluminum ones sure look cooler! 
I would imagine the aluminum bodies to be a little smoother/lower friction than the plastic bodies, so there maybe a subtle performance benefit there. It would really have been nice if they built the shock like a modern top-filled shock though and not like a copy of the xxx-t shocks from 2003 LOL
I was hoping that the aluminum would at least be smoother, -if they are I can not tell a difference. I built the aluminum ones and seriously compressed both the aluminum and plastic in my hands and could not tell a difference at all. The plastic's are fine in my opinion (w/ the exception of the backwards design). The aluminum are just for bling as far as I am concerned. I put them on my mini-t 2, yet to be honest the solid black plastic ones look just as sick ...I kind of even like the solid black a bit better. I have my oil filled TE 18R "factory team" shocks which I pulled out, they are much much nicer than the aluminum Losi ones. In fact they are about the same size and could probably be modded to work ...something I may consider at some point.
I spent $50 for the full set of aluminum shocks, to be honest the $50 could have been spent on an upgraded esc/motor combo ...that would have provided a much better performance increase for the money.