You should always buy in the maximum. If that is 100bb or $200 then you should have the max and learn to play with 100bb. The 50bb buy in is for birds because you are restricted to play a short stack strategy and win another from villain the equivalent of your stack.
If you want to build your bankroll fast you got to play a short stack strategy with a large effective stack of 100bb or more so that you can take another villain's deep stack all at once in a big scoop.
In live, LLSNL games, you win money by doubling up or busting people. Anything else makes you fall victim to the rake.
You have a chance to play a big pot approximate three times per hour. You flop a set, have the nut flush draw, hit your straight... etc. Most of these times a big pot does not pan out. It takes two hands to get paid in a big pot. Or you just miss. Once every other hour, you play a big pot. You are ahead every time. You know you don't always win, but you get your money in good. Most of these you win. On rare occasion, you lose. When you go card dead, when you don’t find money making situations, when the draws go in the muck, when AK misses 2 out of 3 times and your c-bet doesn’t work, your stack and profits slowly melt away until you hit your average hourly rate. That’s the normal way the game goes. There’s nothing magic anyone can do to change those things.
You can play TAG and this is the usual result, three chances an hour and one big pot every other hour. You can play LAG and increase the chances of both, but your swings will be a lot bigger and based upon a lot of observation and experience the results will be the same.
That's the theory.