The only difference I have seen is that Ranked matches have to start w/ 10 people, and after the match is done, it will boot you back to the main menu.
For Public, it can start w/ any combo of bots & gamers, & it can put you into a middle of a game (like CoD). And after the match is over, it will automatically load the next map for continuous play.
In Gears 1 & 2 there was an incentive to play Ranked games, as it was the only way to get achievements and XP, respectively.
But if you can get XP in both, the only reason I see for playing Ranked is to get a more "even" team balance.
Other than this, is there any other subtle differences, like more XP, etc.? If not, I really don't see the point in playing Ranked, as Public is a much more faster process.