Mental Release from Breath Versus Metta
Someone practising metta meditation can make one's mind concentrated easily but metta meditation does not in itself result in deep absorption as the object of concentration is changing and vittaka vicara is needed think "so and so be happy", etc.

Breath meditation, on the other hand, helps develop concentration and studies ones mind from overthinking.

Breath meditation with a bit of metta meditation will help deepen the concentration.