Eh, I side with the Brotherhood because I see them having the potential to do what the NCR is doing, making life more bare-able for wastelanders. BoS protect people, BoS gave out water for free, BoS has all the qualities to help make the world better just like the NCR. Hell, NCR made life so much better for the people in the west that having to carry a gun on your person all the time is no longer needed, settlements don't get attacked hard unless they're on the frontier like the Mojave.
Expanding into territory where only a handful of people live, people who don't have much in their life and live in constant fear of being murderer by their next door neighbor or a raider gang because they have resources said people want. Institute doesn't care to protect people, if they had it their way they would spend all their time underground with the only communication being sending up synths that act more like watchers or people to do experiments. RR doesn't care about helping people or stopping raiders, they only care about freeing people and the MM is only strong if they have a charismatic leader or the castle is still up.
BoS has the potential to help more people out then any other faction from what I've seen, sure MM may have good reasons and may be a good choice but their entire structure is dependent on a central core or a strong leader. RR may want to free people but after that they don't care what happens to said people or what those people turn into and Institute is happy playing "no one's home, go away"
Yes expansionism can be seen as bad but you know what's worse? Starvation, raiders, dehydration, living in fear. I'd rather be under a country that can hold it's own and may be expansionist towards land that is largely unused or uncivilized then "whoops woke up with a knife in my chest and all my possessions gone, screw me I guess"