With PPS, you simply earn a fixed amount based on how many shares you submit to the pool (how much work you submit to the pool. You will generally pay the highest pool fee for this payment method because the risk to the pool is high.
With PPLnS, every "n" shares submitted to the pool by all the miners at the pool, a shift will be complete. Block rewards will be distributed evenly to the last "x" (usually 10) shifts. Many pools offer 0% pool fees with this method, and this is a generally a low variance method, however it does have a higher variance then PPS.
Over the long run, if the pool is big enough, you should expect to earn the same amount. If the pool is too small then variance early on could affect long term results if the difficulty is increasing quickly.