The Alteri Unity is a Psychic race, and was heavily relying on trade as far as I know. However, I seem to believe they have a war going on with a small Empire, the Kilrah. (Granted, they are first, they have the strongest economical base, a fleet to go with, but there is hope. You can hope to survive at least three turns!) So if there is such a war, you will need some pretty good arguments to keep Kilrah at bay. If you are up to the challenge, then definitively join as the Alteri!
Se5a, at least you still have a planet. I know Empires that would kill just to keep a single planet.
(Unless your own planet is on the verge of being destroyed that is.)