Mining, by construction, is a stochastic process. A Poisson Process to be precise. This means that even at a constant difficulty the time between a miner finding a block and the next is randomly distributed. As difficulty rose, this meant that you could go months without finding a block and then have a single afternoon where you found 2 in a row. This variance in the time between finding blocks is intrinsic to how mining works and is independent from any change in the difficulty. It's this variance PPS reduces, it is therefore independent from the difficulty changing.
Mining pools basically gather the computational power of multiple users, and should a single user be lucky, everybody gets their share. The way this share is distributed however is different between the two models you cite. The PPS model has the pool operator take on a certain risk, because he pays you for every share you submit. This means that you as a miner get payouts frequently even if the pools as a whole did not find a block. The other model waits for a block to be found and then distribute the reward among the users that participated.
Basically, if you are mining and are not directly reliant on a regular stream of bitcoins coming in, DGM is the way to go. If you are living on a knifes edge, reliant on a regular flow of bitcoins to keep you going PPS might be for you.
As a personal opinion: if you are serious about mining, go for non-PPS as you don't have to pay for the risk of the operator.