Hi and welcome to the world of frequent flying and BAEC. This the best place at FlyerTalk

First of all I suggest you have a look here: The BA dashboard.

To make things easy for you: Do you fly in economy/coach, or do you get to fly premium cabins like Economy Plus/Economy Extra or in BA lingo World Traveller Plus and finally Business and First.

If yes, then stick to BA as you'll reach Silver in no time with very strong benefits over UA/* Silver. If only coach, then you may gain status quicker on UA. However, I'd rather be stuck in coach on BA than UA!