Guild Wars 2

Can a gem and sigil be applied to a weapon or only 1? Also if it’s only 1 which is more beneficial?

Only one can be applied. Wich one is more beneficial depends on the stats, but in general gems can also go on e.g. an earring, where a only goes on weapons, so they are in general better, for example, an orb (max tier gem) gives +2% crit damage while a superior Sigil of earth (max tier) has a 60% change on applying a 5 seconds bleed when dealing a crit. The bleed is bout 10 times more effective compared to the general +2% damage.

So without the stats and numbers, sigils are more beneficial then gems.