Hmm...the era of Viv Richards and Greg Chappell did correlate, although Richards certainly had the longer career (1974-1991 opposing 1970-1984). That being said, the era itself is not my area of expertise, so I can only go by what I heard.
From what I heard, Viv was a real swashbuckler. He was as infamous for his ODI exploits, if not more so, than his Test exploits. Not only does he have the record for the fastest Test hundred, but his descendants, such as Adam Gilchrist, still spoke of his achievements - such as his ex-MCG record of 153* in 1979, where "he belted Australia from midwicket to deep cover". Gilchrist also said that Viv had "an air of invincibility and absolute certainly that he was about to wreak havoc".
However, Greg Chappell apparently possessed a lot more finesse than Viv Richards. Also, while Richards' impact on proceedings was often diluted by the presence of Gordon Greenidge, Desmond Haynes and even Roy Fredericks at the top of the order (which may've actually boosted his Test/ODI bowlers due to the demoralisation of bowlers), the same could not be said of Chappell (blokes like Ian Davis, Alan Turner and John Dyson batted before him). Also, while Chappell succeeded against everybody he faced up with (he averaged at least 45 against any given country, a terrific achievement, even then), Richards spent much of his career bashing an England attack which was good at times and quite risible at others - he averaged <45 against Australia, New Zealand and Pakistan.
So, as a Test batsman, I'd not hestitate to pick Chappell, if only for his consistency. The opposite applies in the case of ODI's - I have no reason to believe that Chappell was at Richards' level - he lacks his...um...notoriety in that format, for one thing.
Since each was better at one format than the other, I'm gonna go for Brad Haddin.