Your comparing two entirely different apples. An A6 and A8 is an integrated chip with GPU on board. If your computer doesn't have a GPU and you want to save money and maybe do some moderate gaming the A8 is the way to go. If you want to game in high quality or do some serious work get the i3 and a decent GPU. Just expect to pay 4x more for that extra power.
AMD A8-3850 $84.99 Quad core
or
Intel I3-3225 Dual core 144.99
(integrated graphics suck) So you would need a graphics card.