I get the two mixed up. Doesn't really matters what drives them, like I siad I get my info from the games, it was stated by G-man that he wanted to prevent them from reaching their goal and it had to be done soon(which is to control the multiiverse). Also, thing is, any technology you use against the combine is reverse engineered and used against you. Time Lords would be fighting the Time Lords while the Combine sat back and watched.