The candidate who will win the primary, or even whether the two will be in the same district, could be determined by redistricting. Florida has yet to pass its maps, and the Sunshine State is adding two congressional districts next year. It also passed a "Fair Districts" law calling for compact districts that aren't drawn for incumbents. Central Florida is all but certain to gain a district. Whether it is Democratic-leaning or strongly Democratic and heavily minority could influence whether the primary winner is assured a general-election victory.
Demings is an African-American woman. If Republicans create a new Orlando-based district with a large number of minority voters, that could benefit her in the primary and all but guarantee whichever Democrat wins the primary will win the general election. If the district is more suburban and competitive (and less minority-heavy), Grayson's high name recognition and strong fundraising abilities could help him win the primary, making the seat more winnable for a Republican candidate.
Grayson, a favorite of liberal bloggers, is known for his bombastic personality and unbending faith in the liberal cause. He displayed both traits in his announcement email, saying he'd decided to run for Congress again because "a dying man" who had received insufficient care from the Veterans Administration and his wife had asked him to.