In the long-run, both are designed to give the same payout.
PPS places a bunch more risk on the pool, since they must continue to pay out even during really long rounds. That means miners don't bear the risk of not getting paid. Even better, you simply get money sooner, so it's a bit like getting a zero interest loan from the time you submit the share to when a block is found and confirmed.
The main difference is not the long-term payout, but the fees charged by the pool. Deepbit probably has the largest share of the PPS market and their payout is equivalent to a 10% fee. Their proportional fee is 3%, and many newer pools have a 0% fee. If that early payment is valuable, PPS may be worth it for you.
But as a low-hash miner, you won't earn many bitcents a day. You'll spend more time waiting to pay out than you would waiting for your proportional shares to convert.