Whether or not you can get a one-step doesn't just matter on how sick you are; some surgeons will not do them no matter what. The reason is because of the risk of a bowel leak at the anastomosis site (where the pouch is sewed or stapled to the tiny bit of remaining rectum)--or, actually, from the other places where bowel is sewed to bowel during the procedure. When that happens, it creates a very high risk of pouch failure. Once my surgeon (at Mayo) had explained this to me, I knew that i wouldn't want the one-step even if he were willing to do it (and he wasn't).
Surgeons who are willing to do one-steps in general usually won't do them if you've been on prednisone recently (or, if they've been reading the research, on Remicade recently). If you're sick enough to have a colectomy on an emergency basis, that's probably too sick to have it done in one step (and maybe even in two).
The major advantage, obviously, would be avoiding the ileostomy and an additional surgery. But the ileo really isn't as bad as you think.