It could well be Gordon Brown v John Reid with Gord the obvious winner. Bizaarly this would enable Brown to establish himself as the 'break from the Blair era' candidate as against Reid. (No-one remotely interested in politics would fall for it, but a fair number of LP members and MPs might). This worked for John Major, but only because no-one knew who the fuck he was.
John McDonnell could well lose some potential MP votes on the basis of MPs backing Brown against Reid. I have no doubt that anonymous sources close to Blair will give off the record briefings to the press indicating that Reid is the Blair fave. This will be the kiss of death to Reid, although he doesn't need it as he is a dead duck already. But it will give Brown the benefit of being opposed by the Blairite loyalists. In fact if Brown wants to establish himself as a potential General Election winner he desperately needs Blair to denounce him as a far left lunatic peace monger.
Brown could pick up the stop Reid votes in Parliament. McDonnell is really going to struggle to get enough MPs to back him - he really needed to rely on the anti-Blair votes, as very few MPs back his socialist policies. Brown has the albatross around his neck of having been thoroughly implicated in Blairism. But Reid is seen as even more Blairite! McDonnell has more grass roots support then is reflected in the Parliamentary party. This is reflected in the recent votes for the LP NEC, though fewer LP members voted than will in a leadership election. Many LP members don't know what the LP NEC is or does. That includes some LP NEC members. Come to think of it, what does the LP NEC do?
But if McDonnell does get to stand the Trade Union votes will go to Brown unless the unions ballot their membership (if they do ballot then McDonnell could pick up some of the union votes but this is unlikely). Those union leaders who are close to McDonnell are all from non-affiliated unions - RMT were kicked out, FBU left and the PCS have never been an affiliate. E.g. UNISON are likely to pull out all the stops for Brown as presumably they want to see their members pensions robbed.
Brown is of course by far the best looking candidate, and he has a sexy gruff voice.