Metal slimes are nasty at first and don't take any damage, then after you level up a lot they're super easy. Tonberries are horrible unless you cast stop on them. I really don't feel that the two can be compared. Metal slimes aren't even all that bad unless they gang up on you (which they do a lot of), and even then they run away a lot. The only enemy from FF that really comes close are flans, but even then they're just way different.