I am wondering which is better against the undead? Weapons made of silver or magical weapons or does it depend on the particular weapon? When I look at the damage stats, the magical weapons seems to have slightly higher damage stats than the silver ones (example 1d6+4 vs 1d6+3). Would both weapons work against the undead?

I also thought I read somewhere that the undead can only be hurt by silver weapons? Is this true or not & does the same apply to all Gold Box games or is it different for each one?

The weaker undead (skellies, zombies, ghouls) can be hurt by any weapon, while wraiths, wights and spectres are immune. Not sure which ones can be damaged with silver weapons.
Magic weapons are better than silver weapons, since they also give a bonus to hit, and you can be sure they will be effective.