I workflow on a tablet (iPad specifically) is a bit diffrent from a full fledged desktop version. Once you learn the diffrences they can be a powerful tool. Try to get the most screen size for your budget and the most powerful one you can afford. I use a 3rd gen iPad and Sketchbook Pro or iPad mostly. It works well, the canvas is "HD" large in this case and is exportable as a very printable png or PSD file.