On the authority of Aboo Hurairah may Allah be pleased with him said, the Messenger of Allah said

'Al-mu'min al-qawiyy [a strong believer] is better and more beloved to Allah than a weak believer and in both there is goodness..' [Report by Muslim Kitaab al-Qadr no. 02664]

What is meant by a strong believer is in terms of his Eemaan and not in terms of his body, because physical strength might work against a person if he uses the strength in sin and disobedience, so physical strength is not in itself praiseworthy nor disliked but if a person uses his strength in what benefits him in this world and the Here-after then it will become praiseworthy but if he uses his strength in disobeying Allah then it will be considered as a disliked thing [hated]

As for the strength that is mentioned by the Prophet [in the noble hadeeth] refers to strength of Eemaan, this is because the meaning of the word al-qawiyy [strong] is linked to the  first description i.e that of Eemaan [in the statement al-mu'min] just as you might say [in arabic] ar-rajul al-qawiyy [a strong man] refers to strength in his manhood so in the same way, this means strong with regards to his Eemaan.

This is because a believer with strong Eemaan will be able to undertake the obligations that Allah has placed upon him and even increase in voluntary deeds as much as Allah wills [for him]. But as for the person with weak Eemaan then his Eeman will not help him in preforming the obligations that Allah has placed upon him nor stay away from the things Allah has prohibited, thus falling short in many aspects.

The Prophet's statment:

..and in both there is goodness..

Meaning there is goodness in both, the person with strong Eemaan and the one with weak Eemaan. The Messenger of Allah only said this in order to remove doubt least anyone should think that there is no goodness in a person with weak Eemaan, rather there is indeed goodness in a person with weak Eemaan since such a person is better then a non-believer without a doubt.