Ha!
If she does poop, I doubt if anyone would even know it. It would be so small. As far as the ex-pen is concerned, I had one set up just in case I needed to put mine in it while I was busy. I never used it. In fact, I have never used one before, either. There is always someone home, so it serves little purpose for us. We just limit the area. She's house trained now. In fact, she's 8 months old today!
But we still limit the area. Don't want to ruin a good thing.
I'll let someone else answer you on the ex-pen. Some of them are little climbers!
So, IMO which isn't worth much, I'd probably say to use the higher one. You might want to post your question in the training section. I know some people here have had them climb out of the ex-pens and over baby gates, too. In fact, someone had a video on here one day of one of there babies climbing over a baby gate!