The metal ones get WAY hot. Even the plastic ones will get hot, but leave it to the Germans to invent a stamped sheetmetal handguard. I mean, COME ON! All the metal handguards do is conduct the heat right to your hand. Go with the plastic ones if you're ging to shoot it at all.
Weight difference is non-existant between the two. It's not like the steel ones are thick or anything. The steel/wood Israeli handguards weigh a ton, but they also shield from heat no matter how many rounds you fire out of it. They're probably the best handguards for heat protection, but also heavy. The plastic ones are probably the best overall for weight, heat and strength.
Ross