Mindfulness : relation between manasikāra and Citta What is the  correct use of manasikāra ( attention ) and Citta in the context of Mindfulness ?

Is there any difference in terms of spiritual  effect of keeping attention while I do daily works and keeping attention on breath with the mental recitation of "Buddho" ?
Manasikara is a cognitive and thinking function of the mind and related to volition or sankharas. Basically you bring your attention to an object, but this can be tainted by our perception, views and habitual tendencies. Yonisho-manashikara is wise habits of thinking where attention is directed to the source or cause of experience and events. It is a factor of stream-entry (Sotapanna) or first stage of enlightenment. Ayonisho-manasikara is unskillful habits of thinking (paying attention or fixating on unwholesome thoughts, feelings, perception and irrelevant issues)  that perpetuates suffering and entanglements.

Your attention should be on the awareness of the arising and passing within the framework of the 4 foundation of mindfullness, without clinging and craving towards any sense inputs / sensations (equanimous). This way your attention is right. 

When reciting a word it creates verbal fabrications. When you have sloth and torpor then this is fine, to stir up energy. But in general practice may not be the best thing to do, unless you are aware of it's implications. 

When doing daily work be mindful of the breathing, postures by scanning the outline of your body, sensations on your body, as they are generally easy to do.