Yeah, major differences indeed. The plastic takes up the reel after you insert it in enough. Then you twist it until its all in there. All the film is evenly spaced and what not, and you usually get a better result (in my opinion at least).

Metal reels have a clip in the center that you clip your film to. You sorta wrap it around the center, and its supposed to space it evenly for you. But, it you wrap it too tight, the film will touch each other and then no chemicals hit the film. =/ They're trickier, I don't usually like them. A professional photographer called me a puss once because I use plastic reels.
rofl I'm also thinking about shooting like this intentionally; shooting 2 stops under and over processing. I dunno.