It's tough to call. My own expectation is that Miliband will be PM, but in an unstable coalition which won't last.
I too think that is a likely outcome. The only thing that ultimately dissuades me is that the SNP are on course to take 50 odd seats from Labour. I just don't see where Labour will compensate for a loss of seats on that magnitude (and then some more) south of the border. If the SNP only take 20 or 30 seats from Labour then I would say Ed is a shoe-in, but it really does look like Labour are going to be wiped out in Scotland this time around.