question on a 95 small port 3100

[95cutlassproject]

i noticed when i went to the junkyard recently (looking for large port UIM and LIM) i saw that all the other 95 small ports had metal valve covers where as mine engine has plastic. What i want to know is why does mine have plastic valve covers while all the others i saw were metal?, is there any advantage to using the plastic covers? and does having these plastic covers mean i don't have slots for roller rocker. (i will be doing that in the weekend to come if time permits)
Thanks for any answers in advance

[bszopi]

Pick up a set of metal, as the plastic ones tend to leak.