Bankroll vs. Ability

I have been reading that you need a bankroll of 300 times the big blind to account for downswings and variance.

My question is:

If you are not that good wouldn’t variance be a good thing?

I mean if you can have a down swing you could also have an up swing,
so if you are not a very good player would you be better off playing at a 15/30 table with $1,800 or at a 3/6?

My logic is this:

At 3/6 I need to win many more hands to win say $1,000 but at 15/30 or $2,000 NL all I need to do is get lucky a few times or even just once and I could win that amount.

I also read that if you are heads up in a tournament against a weaker player you should try to keep the pots small and gamble less since in time you will out play him, so as the weaker player I would want to do the opposite and make big pots. Right?

So wouldn’t the same hold true for cash games?

Thanks