A lot would depend on the consequences a David Milliband leadership would have on the rise of other parties. The SNP would probably still win a majority, triggering an independence referendum as in OTL, since Scottish politics often bear little relation to what is going on in Westminster, and David Milliband probably wouldnt be all that popular in Scotland anyway. I cant see him playing the referendum any differently, in fact he might be even stronger in joining with the Tories to defend the Union. So there would still be a similar situation in 2015 with regards to a deal with the SNP. That said, maybe he is firmer than his brother from the outset in saying no deal, and it becomes a non issue, it is hard to say.
As for UKIP, they would still probably grow in support, but whether it is to the same extent is another question. On one hand, you could argue that there growth had something to do with the lack of an obvious alternative after the removal of the Lib Dems as the 'protest party'with an uncharismatic and unpopular figure leading the Labour party as well as immigration. In the leadership campaign, he did seek and win the backing of Gillian Duffy, so perhaps he had a better agenda on immigration than Ed. On the other hand, he was hardly the best placed to be on the right end of anti estabilishment politics.
So I would say UKIP would poll a couple of points less than OTL, but that gain would be placated by a growth in support for the Greens. Maybe they would take a couple of seats from them, like Norwich South and Bristol West, but there main effect would be to split the left wing vote in certain places and keep the Tories in some marginals.
By the looks of things he would have done more to address the economic trust issue right from the get go
http://www.theguardian.com/politics/2011/jun/10/david-miliband-labour-leadership-speech
So I reckon he would have a better shot at resolving that issue, but whether that would have been enough is another question. Labour OTL 2015 had pledged an extremely disciplined approach to public spending, but voters still thought they werent trustworthy.
So it is difficult to say precisely how he would do versus his brother, as it is dependent on several uncertain variables. That said, my hunch would be that he would do better, though probably only by 40 seats at best. So best case scenario is that Labour are the second largest party but go into an unstable government with the support of the SNP, who would have a tense relationship with him and pull the plug on them at any time.