I think it is solely up to preference. All of them add new sectors, ships, and missions. All of them except avarice add new weapons (unless you count the yacht's special gun). Can't speak to the integration for existing saves - I would imagine they would. You certainly wouldn't be able to take them back off though.
Pretty much all of them add about the same content and the only real difference is the missions, so I would pick based on your preference for races/ships. As far as the actual missions go I liked Avarice and Terrans the best. Boron is just okay, and the Split campaign is lacking, costs an absolutely obscene amount of money to do and also gives no rewards other than a faction status change.
As far as ships go, Terran ships are basically the best in the game but use a different supply chain, Boron are fast and well shielded but expensive, Split have a lot of guns. Avarice's are good and novel (the L has a lot of unique properties) but again they are the only DLC that does not come with actual new weapons.
