The press, as usual. set Fergie and Diana up as good girl/bad girl so that Diana was seen as acting with decorum, Fergie as a vulgar and gauche show-off. They approved of Diana's looks and fashion sense (which was good, to be fair!) while Fergie was criticised for being fatter (she wasn't fat, of course) and wearing awful clothes.
Both had relationships with other men while married but Fergie always seemed to get caught in highly embarrassing situations - getting her toes sucked by her 'financial advisor' while sunbathing topless with her small daughters. She was persona non grata for a long time after that.
They were good friends, I think, for many years and supported each other through the difficulties of being royal. But a short while before Diana died she fell out with Fergie - and I think it was connected with the veruca issue Roussette mentioned (Fergie borrowed D's shoes and then told a reporter jokingly that she'd caught verucas from them. D was not amused).
From all I've seen, heard and read over the years I get the impression that Fergie was basically a jolly, kind-hearted person with very poor judgement who liked a joke and wasn't manipulative or sulky. Diana could be these things, though, and was capable of bearing a grudge and falling out with folk.
In fact there was a lot of criticism and anti-Diana feeling in the country in the weeks leading up to her death. I remember a mood of 'she's letting herself down by associating with that shady and louche Al Fayed family'. And we know how that ended. But once she was dead the mood changed immediately to one of national hysteria of a very un-British kind.
I was never a massive Diana fan, but I too was moved by her death and the beautiful funeral service. But I also felt that the Queen was bullied into behaving in a certain way and that was quite an ugly feeling. There was a simple-minded attitude at the time that, unless you were openly weeping and letting it all hang out, you somehow didn't care. Not everyone is like that - least of all the Queen! Even if you're not a monarchist, the sight of people being forced to appear and preferably weep in public is distasteful.