I would go with all nights. Also, is the pay better on nights? If so, then for me it would be a no-brainer. :)
I work nights and I LOVE it. I was on days for my training and I hated it. Too many people, too much to do, too much stress, and 15% less pay. No way.
A couple of my family members work some nights, some days, and really have a crazy schedule (they're not in healthcare, though). I was talking to them about how lucky I feel that I only work nights when they never know when they have to work and you know what they told me? They said they felt sorry for ME because with their job, at least some of it is days instead of nights all the time and therefore their sleeping schedule isn't as screwed up as mine! LOL So I guess for some people the day/night thing works better.