Definitely get the other as soon as possible, other than that it depends on the situation. If you need more combat power go with another Halcyon, if you take a while to get your third cap and have some carrier cruisers a Rapture works nicely. If you're playing the Rebellion beta the discord is amazing, but I don't think its a good first cap unless you really have issues with corvette swarms.

I still think mothership is the best, after mothership I get the discord. 

You can also go Halcyon(Dual Halcyon seems less viable since corvettes came to this game). You can then get a mothership after this to.

I also like how Vertigo from the Rapture works with the Titans synergy.