If I had to choose, I would choose Shadow Lands. Bascially what Alien Invasion gives you to ability to build cities, allows you to get great XP from Alien raids, allows you to go onto Alien ships, allows you to get alien perks. And a few other areas to get into. Oh it it gives you a couple new professions.
Shadowlands gives the the ability to access a whole new world, which is very very cool, along with new profesions.
Both give you access to Notum wars perks, and both were worth it to me to buy.
I would suggest however if you going to buy either, let me know I can got mine at the local EB for 5 bucks a peice, you may try to see if your local store has it also.
As for whether they are going to combine these I don't know probably a question for the AO boards.