It's the old problem of "Better in the tent pissing out, then outside the tent pissing in."
If you have Gordon Brown on the backbenches then Brown could - and that is the salient word - have all sorts of shit going on, depending on how Brown played it. I think if he had been sacked he would probably have drifted into irrelevancy eventually, frankly - whatever you want to say, Brown is no Heseltine - but it's hard to tell.
A lot depends on whether Brown would position himself as an out and out anti-Blairite rebel and probably challenge for the leadership, or whether he'd generally be more measured in his behaviour.
Also, a lot depends on the circumstances. If Blair just sacks Brown, then that is going to be quite different from if Brown is forced to resign somehow.