It sucks, man. I thought getting free stuff would be fine, but once it expires, the games you got for free expire too. I was about to be done with Abe's Exodus until it said PS+ expired and I need to buy it to finish it. I considered buying it so I can finish it, but then another thought came into my head: This is a promotion. It gives you something for free, takes it away so you can give them money for it. Also, the discounts aren't worth it. Pisses me off. The worst thing about it is the XBL Gold account makes it inferior in almost every way. They could've improved a few things like making minis free, non-expiring giveaways, or even a free trial of Netflix.
But if you feel you want to buy it, get the 12 month over the 3 month to get the most out of it. The 12 month membership gives you time to finish free games and discounts will begin to add up. The 3 month is a big ripoff.