I don't who devised the saying 'An apple a day keeps doctor away'. All I can guess is he might be marketing expert promoting apples of Jammu Kashmir. If you compare the nutritional facts of apple with amla, you will find that amla is lot more better than apple and lot more cheaper than buying apples @ 120 rs per kg (4-5 pieces).
Compare apple with gooseberry (amla) in this comparison chart.
Apple is good for taste but if you want to live healthy economically then amla turns out to be better option. You can have apple once in a while.