Lily,
I got hired with UAL in 1995. Our class had a United FA hired as well. He was placed first in seniority for our class (others by the last four of their SSN) and he got to keep his company seniority (date hired as a FA) for pass-travel benefits. I'd say if you want a United flying job, that getting hired at United (FA, Gate Agent, etc.) most likely can't hurt and could benefit you in preference hiring and the other benefits above. Of course Continental is now running the show and I don't know what the new hire policies will be when we start hiring off the street in the next years. We also have around 400-600 pilot retirements going forward the next 10 or so years, so good timing for you!