Fumiko
It really depends on what you want for your baby. I don't use the play yard as a safety net for sleeping. It is more of a safer play ground.
These are my conditions when I weigh the pros and cons between a play yard and play pen:
1. He has been crawling very fast so it is tiring running after him so that he won't venture into the "not so child-friendly" places such as kitchen, fan area, table, his changing station etc. So I need some kind of fence. Play yard, play pen and child safety gates are my options.
2. Space. He has a huge play area but he still moves out of the cushioned area. Also, his balls have been rolling around haha, i'm tired of picking the balls for him. If the ball rolls under the sofa, he will also crawl there and pick up dirt on the floor as well.
As play pen is a small area, it does not have space to put his activity table and other stuff and he will be bored and want to get out, just like in his cot. His cot is even bigger than play pen.
3. He has been trying to stand in his cot, so play pen could actually be better since it does not have hard wooden railings which may cause some injury if he bangs hard on them. He has been wriggling in his cot and we have to put cushions round the cot with bumper skirtings as he often bang his head when he rolls around in the cot.
So why choose play yard?
1. Play pen is too small, we think he won't want to stay inside long, just like in his cot.
2. He loves company. Play pen can't accomodate another person.
3. My place has furniture, glass, standing fan etc, so a gate can't fence off all these.
4. He stands and walks around his cot with support on the railings. Play pen is too small an area for him to "walk" around.
5. With play yard, I still can make use of my play mat and those foam yoga mats.
Having said all these, if your gal does not fuss in a play pen, then just stick with it. Saves your money and saves space.
Sorry for the long post ;p