Try them both out on a swatch. Just practice and play around with it. You'll see how they lie. These decreases help to shape an article. Both of these will be used on something like shaping the tops of mittens for instance. Your K2tog will be on the left side of the work and the SSK on the right side. Your mitten top will be rounded and your decreases will look like this:
And if you have trouble remembering which stitch slants which way, here's an easy tip for you: Which way is your right needle pointing after you insert into the loop to do the stitch? When you k2tog, your right needle will be pointing to the right (so the stitch slants to the right), and when you do a ssk, your right needle points to the left (so the stitch will slant to the left).
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