I feel speak the truth and finish it off,
Once I got an offer from a big company (govt undertaking), and i was questioned the same think - why do you want to change the job and I answered - for better prospects. then they asked what better propects do you expect - I said new learning, new exp, expanding knowledge and experience, to this they told - here you will not have any career growth, u have to work for the position always on whcih you r joining (see the demoralisation height), I said - Its ok it will be a new learning as I said and an additional experience, then I got a call from them for second interview and the date is awaited.
This means never lie to employer, speak whatever is the truth, if company deserves you you, they will go ahead or else its ok.