when jekyll feels he must choose, what are the disadvantages he gives for losing either identity? which one does he choose?

In the end Jekyll feels that he simply can't go back. Each time he became Hyde, it became harder to go back. It was kind of like a Faustian bargain or even when Frodo uses his ring (In Lord of the Rings). Jekyll understands that he can't become Hyde. Losing his "good" self means carnage. He also seemed to yearn for his bad side. For a while he could go back and forth. When Hyde began to completely consume him, he takes his own life.