Getting used to sketching digitally took me a long time. It feels very different from sketching with a pencil, that's for sure! I feel most comfortable sketching in SAI, too, out of all of these programs - though that's all I tend to use it for these days, since I've made the switch to Manga Studio.
I do have a cintiq, which helps a lot - but even with that, it took me months to get used to it, so it's not really about which tablet you have, but how much practise you have.