Between the i3 and i5, the i5 should have turbo mode while the i3 doesn't. However, none of these CPU will be bottlenecking anything. The 520m is entry level along with the HD3000 but the 520 has better graphics support and also Optimus so longer battery life. I would go by the price and what you plan to use this laptop for. Both will be fine in daily usage, movies, and casual gaming.